Imaging device and electronic device

ABSTRACT

An imaging device capable of obtaining high-quality imaging data is provided. The imaging device includes a first circuit and a second circuit. The first circuit includes a photoelectric conversion element, a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor, a first capacitor, a second capacitor, and a third capacitor. The second circuit includes an eighth transistor. Variation in threshold voltage of an amplifier transistor (the fifth transistor) included in the first circuit can be compensated.

TECHNICAL FIELD

One embodiment of the present invention relates to an imaging device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, an imaging device, a method for driving any of them, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a storage device, a display device, an imaging device, or an electronic device includes a semiconductor device.

BACKGROUND ART

As a semiconductor device in which pixels each provided with a photosensor are arranged in a matrix, a complementary metal oxide semiconductor (CMOS) image sensor is known. CMOS image sensors are provided in many portable devices such as digital cameras or cellular phones as imaging elements.

Silicon is widely known as a semiconductor material applicable to a transistor generally included in a CMOS image sensor or the like. As another material, an oxide semiconductor has attracted attention.

For example, Patent Document 1 discloses that a transistor including an oxide semiconductor and having extremely low off-state current is used in part of a pixel circuit and a transistor including a silicon semiconductor with which a CMOS circuit can be formed is used in a peripheral circuit, so that an imaging device with high speed operation and low power consumption can be manufactured.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.     2011-119711

DISCLOSURE OF INVENTION

A CMOS image sensor includes an amplifier transistor for outputting data in each pixel. In order to obtain high-quality imaging data, electrical characteristics of the transistors in all the pixels are preferably uninform. However, as miniaturization progresses, the degree of difficulty of a transistor manufacturing process increases, and it is difficult to reduce variation in electrical characteristics.

Output data can be compensated by retaining data for compensating variation in electrical characteristics in a capacitor or the like. However, total imaging time becomes long if data is written to a capacitor by each imaging. In addition, the increase in power consumption becomes a problem.

Thus, an object of one embodiment of the present invention is to provide an imaging device capable of obtaining high-quality imaging data. Another object of one embodiment of the present invention is to provide an imaging device capable of compensating variation in threshold voltage of an amplifier transistor included in a pixel circuit. Another object of one embodiment of the present invention is to provide a low-power imaging device. Another object of one embodiment of the present invention is to provide an imaging device that is suitable for high-speed operation. Another object of one embodiment of the present invention is to provide an imaging device with high sensitivity. Another object of one embodiment of the present invention is to provide an imaging device with a wide dynamic range. Another object of one embodiment of the present invention is to provide an imaging device with high resolution. Another object of one embodiment of the present invention is to provide an imaging device formed at low cost. Another object of one embodiment of the present invention is to provide an imaging device with high reliability. Another object of one embodiment of the present invention is to provide a novel imaging device or the like. Another object of one embodiment of the present invention is to provide a novel semiconductor device or the like.

The description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

One embodiment of the present invention relates to an imaging device capable of compensating variation in threshold voltage of an amplifier transistor included in a pixel circuit.

One embodiment of the present invention is an imaging device that includes a first circuit and a second circuit. The first circuit includes a photoelectric conversion element, a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor, a first capacitor, a second capacitor, and a third capacitor. The second circuit includes an eighth transistor. One terminal of the photoelectric conversion element is electrically connected to one of a source and a drain of the first transistor. The other of the source and the drain of the first transistor is electrically connected to one of a source and a drain of the second transistor. The other of the source and the drain of the first transistor is electrically connected to one terminal of the first capacitor. One of a source and a drain of the third transistor is electrically connected to the other terminal of the first capacitor. The other terminal of the first capacitor is electrically connected to one terminal of the second capacitor. One of a source and a drain of the fourth transistor is electrically connected to the other terminal of the second capacitor. The other of the source and the drain of the fourth transistor is electrically connected to one of a source and a drain of the fifth transistor. One of a source and a drain of the sixth transistor is electrically connected to one of the source and the drain of the fifth transistor. One terminal of the third capacitor is electrically connected to the other terminal of the second capacitor. The other terminal of the third capacitor is electrically connected to the other of the source and the drain of the sixth transistor. A gate of the sixth transistor is electrically connected to one terminal of the third capacitor. One of a source and a drain of the seventh transistor is electrically connected to the other of the source and the drain of the sixth transistor. The other of the source and the drain of the seventh transistor is electrically connected to one of a source and a drain of the eighth transistor.

The second circuit may further include a ninth transistor. One of a source and a drain of the ninth transistor may be electrically connected to the other of the source and the drain of the eighth transistor. A gate of the ninth transistor may be electrically connected to a gate of the eighth transistor. The gate of the ninth transistor may be electrically connected to the other of the source and the drain of the ninth transistor.

The other of the source and the drain of the third transistor may be electrically connected to the other terminal of the photoelectric conversion element.

The first circuit may further include a fourth capacitor. One terminal of the fourth capacitor may be electrically connected to one of the source and the drain of the third transistor. The other terminal of the fourth capacitor may be electrically connected to the other of the source and the drain of the fifth transistor.

Each of the first to ninth transistors preferably includes an oxide semiconductor in an active layer, and the oxide semiconductor preferably includes In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf).

According to one embodiment of the present invention, an imaging device capable of obtaining high-quality imaging data can be provided. An imaging device capable of compensating variation in electrical characteristics of an amplifier transistor included in a pixel circuit can be provided. A low-power imaging device can be provided. An imaging device that is suitable for high-speed operation can be provided. An imaging device with high sensitivity can be provided. An imaging device with a wide dynamic range can be provided. An imaging device with high resolution can be provided. An imaging device formed at low cost can be provided. An imaging device with high reliability can be provided. A novel imaging device or the like can be provided. A novel semiconductor device or the like can be provided.

The description of these effects does not disturb the existence of other effects. In one embodiment of the present invention, there is no need to obtain all the effects. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates circuits of an imaging device;

FIG. 2 illustrates a circuit of an imaging device;

FIG. 3 is a timing chart illustrating compensation operation;

FIG. 4 illustrates compensation operation;

FIG. 5 illustrates compensation operation;

FIG. 6 illustrates compensation operation;

FIG. 7 illustrates compensation operation;

FIG. 8 illustrates compensation operation;

FIGS. 9A and 9B illustrate circuits of an imaging device;

FIGS. 10A and 10B are cross-sectional views of imaging devices including circuit portions;

FIGS. 11A1, 11A2, 11A3, 11B1, 11B2, and 11B3 illustrate a bent imaging device;

FIG. 12 illustrates the structure of an imaging device;

FIGS. 13A and 13B are timing charts illustrating operations of a global shutter system and a rolling shutter system, respectively;

FIGS. 14A and 14B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 15A and 15B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 16A and 16B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 17A and 17B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 18A and 18B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 19A and 19B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 20A to 20D each illustrate a cross section of a transistor in a channel width direction;

FIGS. 21A to 21F each illustrate a cross section of a transistor in a channel length direction;

FIGS. 22A to 22C are a top view and cross-sectional views illustrating a semiconductor layer;

FIGS. 23A to 23C are a top view and cross-sectional views illustrating a semiconductor layer;

FIGS. 24A and 24B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 25A and 25B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 26A and 26B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 27A and 27B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 28A and 28B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 29A and 29B are a top view and a cross-sectional view illustrating a transistor;

FIGS. 30A to 30D each illustrate a cross section of a transistor in a channel width direction;

FIGS. 31A to 31F each illustrate a cross section of a transistor in a channel length direction;

FIGS. 32A and 32B are top views illustrating transistors;

FIGS. 33A to 33C illustrate a method for manufacturing a transistor;

FIGS. 34A to 34C illustrate a method for manufacturing a transistor;

FIGS. 35A to 35C illustrate a method for manufacturing a transistor;

FIGS. 36A to 36C illustrate a method for manufacturing a transistor;

FIGS. 37A to 37C illustrate a method for manufacturing a transistor;

FIGS. 38A to 38C illustrate a method for manufacturing a transistor;

FIGS. 39A to 39F illustrate electronic devices;

FIG. 40 is a timing chart illustrating operation of an imaging device;

FIG. 41 is a timing chart illustrating operation of an imaging device;

FIG. 42 illustrates a circuit of an imaging device;

FIG. 43 is a block diagram illustrating an imaging device;

FIG. 44 is a circuit diagram illustrating a motion detector;

FIG. 45 is a timing chart illustrating operation of a motion detector;

FIG. 46 is a circuit diagram illustrating a pixel including a threshold voltage compensation circuit;

FIG. 47 shows timing charts illustrating motion capturing operation;

FIG. 48 is a photograph showing the appearance of an imaging device;

FIG. 49 illustrates output distribution of all pixels;

FIG. 50 illustrates variation in average output between pixels;

FIG. 51 shows images taken by an imaging device;

FIG. 52 illustrates a motion capturing range ΔV_(REF) in average output of all pixels; and

FIG. 53 illustrates motion capturing operation with and without threshold voltage compensation operation.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to the drawings. Note that the present invention is not limited to the following description. It will be readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and scope of the present invention. The present invention therefore should not be construed as being limited to the following description of the embodiments. In structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and the description thereof is not repeated in some cases. The same components are denoted by different hatching patterns in different drawings, or the hatching patterns are omitted in some cases.

For example, in this specification and the like, an explicit description “X and Y are connected” means that X and Y are electrically connected, X and Y are functionally connected, and X and Y are directly connected. Accordingly, without being limited to a predetermined connection relationship, for example, a connection relationship shown in drawings or texts, another connection relationship is included in the drawings or the texts.

Here, each of X and Y denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer).

Examples of the case where X and Y are directly connected include the case where an element that enables electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) is not connected between X and Y, and the case where X and Y are connected without the element that enables electrical connection between X and Y provided therebetween.

For example, in the case where X and Y are electrically connected, one or more elements that enable electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) can be connected between X and Y. Note that the switch is controlled to be turned on or off. That is, the switch is conducting or not conducting (is turned on or off) to determine whether current flows therethrough or not. Alternatively, the switch has a function of selecting and changing a current path. Note that the case where X and Y are electrically connected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, one or more circuits that enable functional connection between X and Y (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit; a signal converter circuit such as a D/A converter circuit, an A/D converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power supply circuit (e.g., a step-up circuit or a step-down circuit) or a level shifter circuit for changing the potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a storage circuit; or a control circuit) can be connected between X and Y. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is provided between X and Y, X and Y are functionally connected. The case where X and Y are functionally connected includes the case where X and Y are directly connected and X and Y are electrically connected.

Note that in this specification and the like, an explicit description “X and Y are electrically connected” means that X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween). That is, in this specification and the like, the explicit description “X and Y are electrically connected” is the same as the explicit description “X and Y are connected.”

For example, the case where a source (or a first terminal or the like) of a transistor is electrically connected to X through (or not through) Z1 and a drain (or a second terminal or the like) of the transistor is electrically connected to Y through (or not through) Z2, or the case where a source (or a first terminal or the like) of a transistor is directly connected to part of Z1 and another part of Z1 is directly connected to X while a drain (or a second terminal or the like) of the transistor is directly connected to part of Z2 and another part of Z2 is directly connected to Y, can be expressed by using any of the following expressions.

The expressions include, for example, “X, Y, a source (or a first terminal or the like) of a transistor, and a drain (or a second terminal or the like) of the transistor are electrically connected to each other, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in that order,” “a source (or a first terminal or the like) of a transistor is electrically connected to X, a drain (or a second terminal or the like) of the transistor is electrically connected to Y, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in that order,” and “X is electrically connected to Y through a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are connected in that order.” When the connection order in a circuit structure is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope.

Other examples of the expressions include “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least a first connection path, the first connection path does not include a second connection path, the second connection path is a path between the source (or the first terminal or the like) of the transistor and a drain (or a second terminal or the like) of the transistor, Z1 is on the first connection path, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least a third connection path, the third connection path does not include the second connection path, and Z2 is on the third connection path.” It is also possible to use the expression “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least Z1 on a first connection path, the first connection path does not include a second connection path, the second connection path includes a connection path through the transistor, a drain (or a second terminal or the like) of the transistor is electrically connected to Y through at least Z2 on a third connection path, and the third connection path does not include the second connection path.” Still another example of the expressions is “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least Z1 on a first electrical path, the first electrical path does not include a second electrical path, the second electrical path is an electrical path from the source (or the first terminal or the like) of the transistor to a drain (or a second terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least Z2 on a third electrical path, the third electrical path does not include a fourth electrical path, and the fourth electrical path is an electrical path from the drain (or the second terminal or the like) of the transistor to the source (or the first terminal or the like) of the transistor.” When the connection path in a circuit structure is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope.

Note that these expressions are examples and there is no limitation on the expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer).

Even when independent components are electrically connected to each other in a circuit diagram, one component has functions of a plurality of components in some cases. For example, when part of a wiring also functions as an electrode, one conductive film functions as the wiring and the electrode. Thus, the term “electrical connection” in this specification also means such a case where one conductive film has functions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with each other depending on circumstances or conditions. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. In addition, the term “insulating film” can be changed into the term “insulating layer” in some cases.

Embodiment 1

In this embodiment, an imaging device that is one embodiment of the present invention is described with reference to drawings.

An imaging device in one embodiment of the present invention includes a pixel circuit that can compensate variation in threshold voltage of a source follower amplifier transistor in a pixel of an imaging device that outputs signal charge (data) by a source follower.

FIG. 1 is a circuit diagram of a circuit 11 that can function as a pixel circuit and a circuit 12 that can function as a reference current source circuit included in an imaging device in one embodiment of the present invention. In FIG. 1 and the like, transistors are n-ch transistors; however, one embodiment of the present invention is not limited thereto. The transistors may be p-ch transistors by reversing the magnitude relationship of a potential as illustrated in FIG. 42. Alternatively, some of the n-ch transistors may be replaced with p-ch transistors.

The circuit 11 is broadly divided into a photoelectric conversion portion and a signal generation portion. The photoelectric conversion portion includes a photodiode 60, a transistor 51, and a transistor 52. The signal generation portion includes a transistor 53, a transistor 54, a transistor 55, a transistor 56, a transistor 57, a capacitor C1, a capacitor C2, a capacitor C3, and a capacitor C4. Note that the capacitor C4 can be omitted.

The circuit 12 includes a transistor 58 and an output terminal (OUT).

The circuit 12 connected to a wiring 31 can have a structure illustrated in FIG. 2. In FIG. 2, a transistor 59 is added to the circuit 12. This circuit is a current mirror circuit.

In the circuit 11 in FIG. 1, one terminal of the photodiode 60 is electrically connected to one of a source and a drain of the transistor 51. The other of the source and the drain of the transistor 51 is electrically connected to one of a source and a drain of the transistor 52. The other of the source and the drain of the transistor 51 is electrically connected to one terminal of the capacitor C1. One of a source and a drain of the transistor 53 is electrically connected to the other terminal of the capacitor C1. The other terminal of the capacitor C1 is electrically connected to one terminal of the capacitor C2. One of a source and a drain of the transistor 54 is electrically connected to the other terminal of the capacitor C2. The other of the source and the drain of the transistor 54 is electrically connected to one of a source and a drain of the transistor 55. One of a source and a drain of the transistor 56 is electrically connected to one of the source and the drain of the transistor 55. One terminal of the capacitor C3 is electrically connected to the other terminal of the capacitor C2. The other terminal of the capacitor C3 is electrically connected to the other of the source and the drain of the transistor 56. A gate of the transistor 56 is electrically connected to one terminal of the capacitor C3. One of a source and a drain of the transistor 57 is electrically connected to the other of the source and the drain of the transistor 56. The other of the source and the drain of the transistor 57 is electrically connected to one of a source and a drain of the transistor 58.

The other terminal of the photodiode 60 is electrically connected to a wiring 21 (VPD). The other of the source and the drain of the transistor 52 is electrically connected to a wiring 22 (VPR). The other of the source and the drain of the transistor 55 is electrically connected to a wiring 23 (VPI). A gate of the transistor 51 is electrically connected to a wiring 25 (TX). A gate of the transistor 52 is electrically connected to a wiring 26 (PR). A gate of the transistor 53 is electrically connected to a wiring 27 (W). A gate of the transistor 54 is electrically connected to a wiring 28 (AZ). A gate of the transistor 55 is electrically connected to a wiring 29 (PSW). A gate of the transistor 57 is electrically connected to a wiring 30 (SE). The other of the source and the drain of the transistor 57 is electrically connected to the wiring 31.

In the circuit 12, one of the source and the drain of the transistor 58 is electrically connected to the wiring 31, and the other of the source and the drain of the transistor 58 is electrically connected to a wiring 24 (VPO). A gate of the transistor 58 is electrically connected to a wiring 32 (BR). The output terminal (OUT) is provided in one of the source and the drain of the transistor 58.

Here, the wiring 21 (VPD), the wiring 22 (VPR), the wiring 23 (VPI), and the wiring 24 (VPO) can function as power supply lines. The wiring 25 (TX), the wiring 26 (PR), the wiring 27 (W), the wiring 28 (AZ), the wiring 29 (PSW), the wiring 30 (SE), the wiring 31, and the wiring 32 (BR) can function as signal lines.

In FIG. 1, the other of the source and the drain of the transistor 53 is connected to the wiring 21 (VPD); however, the other of the source and the drain of the transistor 53 may be connected to a wiring capable of supplying another fixed potential.

In addition, in FIG. 1, the other terminal of the capacitor C4 is connected to the wiring 23 (VPI); however, the other terminal of the capacitor C4 may be connected to a wiring capable of supplying another fixed potential.

In the above structure, a node to which the other of the source and the drain of the transistor 51, one of the source and the drain of the transistor 52, and one terminal of the capacitor C1 are connected is denoted by FD1.

A node to which one of the source and the drain of the transistor 53, the other terminal of the capacitor C1, one terminal of the capacitor C2, and one terminal of the capacitor C4 are connected is denoted by FD2.

A node to which one of the source and the drain of the transistor 54, the other terminal of the capacitor C2, one terminal of the capacitor C3, and the gate of the transistor 56 are connected is denoted by AG.

A node to which the other of the source and the drain of the transistor 56, the other terminal of the capacitor C3, and one of the source and the drain of the transistor 57 are connected is denoted by AS.

A diode element formed using a silicon substrate with a pn junction or a pin junction can be used as the photodiode 60. Alternatively, a pin diode element formed using an amorphous silicon film, a microcrystalline silicon film, or the like may be used. Note that although the circuit 11 includes the photodiode, the circuit 11 may include another photoelectric conversion element. For example, a diode-connected transistor may be used. Alternatively, a variable resistor or the like utilizing a photoelectric effect may be formed using silicon, germanium, selenium, or the like.

Alternatively, a photoelectric conversion element that includes selenium utilizing a phenomenon called avalanche multiplication may be used. In the photoelectric conversion element, a highly sensitive sensor in which the amount of amplification of electrons with respect to the amount of incident light is large can be obtained.

Amorphous selenium or crystalline selenium can be used as a selenium-based material. Crystalline selenium can be obtained by, for example, depositing amorphous selenium and then performing heat treatment. When the crystal grain size of crystalline selenium is smaller than a pixel pitch, variation in characteristics between pixels can be reduced.

In the circuit 11, the photodiode 60 is a light-receiving element and can have a function of generating current based on the amount of light incident on the circuit 11. The transistor 51 can have a function of controlling charge accumulation in the node FD1 performed by the photodiode 60. The transistor 52 can have a function of executing operation of resetting the potential of the node FD1. The transistor 53 can have a function of executing operation of resetting the potential of the node FD2. The transistors 54 and 55 can each have a function of supplying current to the transistor 56. The transistor 56 can have a function of executing operation of outputting a signal based on the potential of the node AG. The transistor 57 can have a function of executing operation of controlling selection of the circuit 11 (pixel circuit) at the time of reading.

In the circuit 12, the transistor 58 can function as a current source transistor. In addition, the transistor 58 can have a function of outputting a voltage signal based on current supplied to the transistor 58 from the output terminal (OUT).

In the imaging device in one embodiment of the present invention with the above structure, an output signal can be compensated when the circuit 11 stores the threshold voltage of the transistor 56 included in the circuit 11.

Details of the compensation operation and output operation after compensation are described with reference to a timing chart in FIG. 3. The timing chart in FIG. 3 shows the potentials of the wiring 24 (VPO), the wiring 25 (TX), the wiring 26 (PR), the wiring 27 (W), the wiring 28 (AZ), the wiring 29 (PSW), the wiring 30 (SE), the wiring 32 (BR), the node FD1, the node FD2, the node AG, and the node AS. Note that each transistor is turned on or off in accordance with a potential supplied to a wiring connected to the gate of each transistor.

In the circuit diagram used for the description, transistors other than the transistor 56 are described as switches in order to clarify conduction state of the transistors. In addition, some reference numerals are omitted. Here, the wiring 21 (VPD) has a low potential (“GND”), the wiring 22 (VPR) has a high potential (“VPR”), and the wiring 23 (VPI) has a high potential (“VPI”).

At time T1, the transistors 52, 53, 54, 55, 57, and 58 are turned on and the transistor 51 is turned off. The potential of the wiring 24 (VPO) is increased to “V_(α),” and the potential of the wiring 32 (BR) is increased to “BR+V_(β).” For example, “V_(α)” can be equal to “VPI/2” when GND is 0 V. At this time, comparatively high bias current flows to the transistor 58, and the source potential of the transistor 56 (the potential of the node AS) can be lower than the gate potential of the transistor 56 (the potential of the node AG)−threshold voltage (V_(th)) (see a current path indicated by a broken line in FIG. 4). In other words, the potential relationship in the circuit is the node AS<the node AG−V_(th).

Here, the potential of the node FD1 is set to “VPR,” i.e., the potential of the wiring 22 (VPR). The potential of the node FD2 is set to “GND” (for example, 0 V), i.e., the potential of the wiring 21 (VPD). The potential of the node AG is set to“VPI,” i.e., the potential of the wiring 23 (VPI). The potential of the node AS is set to “VPO,” i.e., the potential of the wiring 24 (VPO).

Next, at time T2, the transistor 55 is turned off to shut off a bias current path and to discharge the node AG (see FIG. 5). At this time, the potential of the node AG is changed from “VPI” to “VPO+V_(th)” and discharging is terminated.

Then, at time T3, the transistor 54 is turned off to hold the threshold voltage (V_(th)) of the transistor 56 in the capacitor C3 (see FIG. 6).

At time T4, after the transistor 57 is turned off, compensation is terminated by turning on the transistor 55, turning off the transistor 53, and setting the potential of the wiring 32 (BR) to “BR” (see FIG. 7).

Next, on the assumption of actual imaging operation, operation when the potential of the node FD2 is changed by V_(α) is described. First, in order to change the potential of the node FD2 by V_(α), the transistor 52 is turned off, the transistor 51 is turned on while the potential “VPR” of the wiring 22 (VPR) is held in the node FD1, and charge corresponding to V_(α) is discharged to the wiring 21 (VPD) through the photodiode 60 irradiated with light. Then, the transistor 51 is turned off to hold the potential of the node FD1. Through the above operation, the potential of the node FD1 can be changed from “VPR” to “VPR−V_(α).”

When the potential of the node FD1 is changed from “VPR” to “VPR−V_(α),” the potential of the node FD2 is changed from “VPD (=0 V)” to “−V_(α).” In addition, the potential of the node AG is changed from “VPO+V_(th)” to “VPO−V_(α)+V_(th).”

Here, current based on the potential of the wiring 32 (BR) can be supplied to the transistor 58, and bias current is supplied to the transistor 56 when the transistor 57 is turned on. When a difference between a gate potential and a source potential for supplying the bias current to the transistor 56 at this time is denoted by “V_(gs),” (=V_(th)+V_(gs)), the potential of the node AS is (VPO−V_(α)+V_(th))−(V_(th)+V_(gs))=VPO−V_(α)−V_(gs) (see FIG. 8). The following drain current equation (β is a constant) indicates that the drain current (I_(d)) supplied to the transistor 56 does not depend on the threshold voltage (V_(th)).

$\begin{matrix} {\quad\begin{matrix} {\quad{{Id} = {{\beta/2}\left( {{V{gs}}^{\prime} - {V\; {th}}} \right)^{2}}}} \\ {= {{\beta/2}\left( {{V\; {th}} + {V{gs}} - {Vth}} \right)^{2}}} \\ {= {{\beta/2} \cdot {Vgs}^{2}}} \end{matrix}} & (1) \end{matrix}$

Thus, a signal that does not include variation in the threshold voltage of the transistor 56 can be output to the output terminal (OUT), and high-quality imaging data can be obtained. Note that in the circuit structure of FIG. 1, as the intensity of light delivered to the photodiode 60 becomes higher, a signal output from the output terminal (OUT) becomes smaller.

It is not necessary to perform the compensation operation by each imaging and imaging can be successively performed only by one compensation operation. Needless to say, the compensation operation may be performed before imaging, after imaging, at the time of power-on, at the time of power-off, or at given timing using a timer or the like.

An imaging device in one embodiment of the present invention may have a structure in FIG. 9A or 9B. The connection direction of the photodiode 60 of the photoelectric conversion portion in the circuit 11 in FIG. 9A is opposite to that in FIG. 1. In that case, the wiring 21 (VPD) has a high potential and the wiring 22 (VPR) has a low potential. The circuit description in FIG. 1 can be referred to for compensation operation and output operation. In that case, as the intensity of light delivered to the photodiode 60 becomes higher, the potential of the node FD1 becomes higher. Thus, in the circuit structure of FIG. 9A, as the intensity of light delivered to the photodiode 60 becomes higher, a signal output from the output terminal (OUT) becomes larger.

In FIG. 9B, the transistor 52 is omitted from the circuit 11 in FIG. 1. In that case, the wiring 21 (VPD) can be changed to either a low potential or a high potential. FD1 reset operation can be performed when the wiring 21 (VPD) has a high potential. In a predetermined period, when the wiring 21 (VPD) has a high potential, forward bias is applied to the photodiode 60. Thus, the potential of the node FD1 can be set to the potential of the wiring 21 (VPD).

In the case where light detection operation (accumulation operation) is performed, the potential of the wiring 21 (VPD) is set to a low potential. When the wiring 21 (VPD) has a low potential, reverse bias is applied to the photodiode 60; thus, charge can be released from the node FD1 to the wiring 21 (VPD) in accordance with light intensity. In that case, as the intensity of light delivered to the photodiode 60 becomes higher, the potential of the node FD1 becomes lower. Thus, in the circuit structure of FIG. 9B, as the intensity of light delivered to the photodiode 60 becomes higher, a signal output from the output terminal (OUT) becomes smaller.

In the imaging device in one embodiment of the present invention, a transistor including an oxide semiconductor is preferably used. The use of the transistor including an oxide semiconductor in the circuit 11 can broaden the dynamic range of imaging. In the circuit structure in FIG. 1, when the intensity of light entering the photodiode 60 is high, the potential of the node AG becomes lower. Since the transistor including an oxide semiconductor has extremely low off-state current, current based on the potential of the node AG (the gate potential of the transistor 56) can be accurately output even when the gate potential is extremely low. Thus, it is possible to broaden the detectable range of illuminance, i.e., the dynamic range.

A period during which charge can be held in the node FD1, the node FD2, the node AG, and the node AS can be extremely long owing to the low off-state current of the transistor including an oxide semiconductor. Thus, a global shutter system, in which charge accumulation operation is performed in all the pixels substantially at the same time, can be used without a complicated circuit structure and operation method. Therefore, an image with little distortion can be easily obtained even in the case of a moving object. Furthermore, exposure time (a period of performing charge accumulation operation) can be long; thus, the imaging device is suitable for imaging even in a low illuminance environment.

A transistor connected to any of the node FD1, the node FD2, the node AG, and the node AS needs to be a transistor with low noise. The channel of a transistor including two or three oxide semiconductor layers to be described later is a buried channel, which has significantly high resistance to noise. Thus, the use of the transistor leads to an image with little noise.

In the one embodiment of the present invention, an output signal that does not depend on variation in the threshold voltage (V_(th)) of the amplifier transistor (the transistor 56) included in the pixel circuit can be obtained.

FIG. 10A is an example of a cross-sectional view of an imaging device including a circuit portion. A circuit portion 90 is a combination of a transistor 70 that includes an active region in a silicon substrate 40 and a transistor 71 that includes an oxide semiconductor as an active layer, and can form, for example, an inverter circuit or a memory circuit. In addition, a circuit portion 92 is a combination of the photodiode 60 formed using the silicon substrate 40 and the transistor 51 that includes an oxide semiconductor as an active layer, and corresponds to part of the photoelectric conversion portion of the circuit 11 in FIG. 1. Note that wirings and contact plugs indicated by broken lines show that placement is different from that of other wirings and contact plugs in a depth direction.

In FIG. 10A, the photodiode 60 and the transistor 51 can be formed to overlap with each other; thus, the integration degree of pixels can be increased. In other words, the resolution of the imaging device can be increased. Furthermore, since the silicon substrate 40 is not provided with a transistor formed in the occupation area of the circuit portion 92, the area of the photodiode can be large. Thus, an image with low noise can be obtained even in a low illuminance environment.

Although FIGS. 10A and 10B illustrate a structure in which the photodiode 60 and the transistor 70 are formed using the silicon substrate 40, one embodiment of the present invention is not limited thereto. For example, the transistor 70 may be formed using the silicon substrate 40 and a photodiode formed using another substrate may be attached. Alternatively, the transistor 70 may be formed without the use of the silicon substrate 40, and a transistor that includes an oxide semiconductor as an active layer may be provided as in the transistors 71 and 51. Alternatively, as illustrated in FIG. 10B, the transistors 70 and 51 may be provided using the silicon substrate 40. An element other than the transistor 70 may be formed using the silicon substrate 40. For example, a capacitor, a diode, or a resistor may be formed using the silicon substrate 40.

In the structure in FIG. 10A, an insulating layer 80 is provided between a region including the transistor 70 and the photodiode 60 and a region including the transistors 71 and 51.

Dangling bonds of silicon are terminated with hydrogen in insulating layers provided in the vicinity of the active region of the transistor 70. Therefore, hydrogen has an effect of improving the reliability of the transistor 70. Meanwhile, hydrogen in insulating layers provided in the vicinities of oxide semiconductor layers that are the active layers of the transistors 51 and 71 and the like causes generation of carriers in the oxide semiconductors. Therefore, hydrogen might reduce the reliability of the transistors 51 and 71 and the like. Consequently, in the case where one layer that includes a transistor including a silicon-based semiconductor material and the other layer that includes the transistor including an oxide semiconductor are stacked, it is preferable that the insulating layer 80 having a function of preventing diffusion of hydrogen be provided between these layers. Hydrogen is confined in the one layer by the insulating layer 80, so that the reliability of the transistor 51 can be improved. Furthermore, diffusion of hydrogen from the one layer to the other layer is inhibited, so that the reliability of the transistors 51 and 71 and the like can be improved.

The insulating layer 80 can be formed using aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, or yttria-stabilized zirconia (YSZ), for example.

The silicon substrate 40 is not limited to a bulk silicon substrate and may be an SOI substrate. Furthermore, the silicon substrate 40 can be replaced with a substrate made of germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, or an organic semiconductor, or a substrate over which a thin film of the material is formed.

The transistor 70 can be a transistor of various types without being limited to a planar type transistor. For example, the transistor 70 can be a fin-type transistor or a tri-gate transistor.

The transistor 51 can include various types of semiconductors as well as an oxide semiconductor depending on conditions. For example, the transistor 51 can include silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, or an organic semiconductor.

As illustrated in FIGS. 11A1 and 11B1, the imaging device may be bent. FIG. 11A1 illustrates a state in which the imaging device is bent in the direction of dashed-two dotted line X1-X2. FIG. 11A2 is a cross-sectional view illustrating a portion indicated by dashed-two dotted line X1-X2 in FIG. 11A1. FIG. 11A3 is a cross-sectional view illustrating a portion indicated by dashed-two dotted line Y1-Y2 in FIG. 11A1.

FIG. 11B1 illustrates a state where the imaging device is bent in the direction of dashed-two dotted line X3-X4 and the direction of dashed-two dotted line Y3-Y4. FIG. 11B2 is a cross-sectional view illustrating a portion indicated by dashed-two dotted line X3-X4 in FIG. 11B1. FIG. 11B3 is a cross-sectional view illustrating a portion indicated by dashed-two dotted line Y3-Y4 in FIG. 11B1.

Bending the imaging device can reduce field curvature and astigmatism. Thus, the optical design of lens and the like, which are used in combination of the imaging device, can be facilitated. For example, the number of lenses used for aberration correction can be reduced; accordingly, the size or weight of semiconductor devices including the imaging device can be easily reduced. In addition, the quality of a captured image can be improved.

In this embodiment, one embodiment of the present invention has been described. Other embodiments of the present invention are described in the other embodiments. Note that one embodiment of the present invention is not limited thereto. Although an example in which one embodiment of the present invention is applied to an imaging device is described, one embodiment of the present invention is not limited thereto. Depending on circumstances or conditions, one embodiment of the present invention is not necessarily applied to an imaging device. One embodiment of the present invention may be applied to a semiconductor device with an another function, for example. Although examples in which, in one embodiment of the present invention, a function of compensating variation or degradation in electrical characteristics of a transistor is provided or compensation operation is performed, one embodiment of the present invention is not limited thereto. Depending on circumstances or conditions, one embodiment of the present invention does not necessarily compensate variation or degradation in electrical characteristics of a transistor.

This embodiment can be combined with any of the structures described in the other embodiments or Example as appropriate.

Embodiment 2

In this embodiment, an example of a method for driving a pixel circuit is described.

The pixel circuit described in Embodiment 1 can perform first operation in which normal imaging is performed and second operation in which differential data of imaging data of an initial frame and imaging data of a current frame can be retained and a signal based on the differential data can be output. In the second operation, differential data can be output without a comparison process or the like in an external circuit; thus, the pixel circuit can be applied to a low-power security camera or the like.

As illustrated in FIG. 12, an imaging device in one embodiment of the present invention includes a pixel portion 400 that includes the circuits 11 arranged in a matrix, a row driver 410 connected to the circuits 11, the circuits 12 connected to the circuits 11, an A/D converter 420 connected to the circuits 12, and a column driver 430 connected to the A/D converter 420.

Imaging data obtained in the circuit 11 selected by the row driver 410 is input to the A/D converter 420 through the circuit 12. The A/D converter 420 converts input imaging data into digital data by A/D conversion. The A/D converted digital data are sequentially extracted to the outside by the column driver 430. As the row driver 410 and the column driver 430, for example, a variety of circuits such as a decoder and a shift register can be used.

Next, first operation of the circuit in FIG. 1 is described with reference to a timing chart in FIG. 40.

From the time T1 to the time T2, the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to a potential higher than VPR+V_(th), and the wiring 27 (W) is set to a potential higher than V_(th). At this time, the potential of the node FD1 is set to the potential of the wiring 22 (VPR) and the potential of the node FD2 is set to the potential of the wiring 21 (VPD), i.e., “GND” (reset operation).

From the time T2 to the time T3, the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential lower than −VPR. Here, the potentials of the node FD1 and the node FD2 are decreased in response to light with which the photodiode 60 is irradiated. When the amount of decrease in the potential of the node FD1 at the time T3 is denoted by V1, the potential of the node FD1 is VPR−V1. In addition, the potential of the node FD2 is decreased by V2 due to capacitive coupling and becomes GND−V2 (accumulation operation). Note that in the circuit structure in FIG. 1, as the intensity of light delivered to the photodiode 60 becomes higher, the potentials of the node FD1 and the node FD2 become lower.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential lower than −VPR from the time T3 to the time T4, the potentials of the node FD1 and the node FD2 are held.

In the case where the wiring 30 (SE) is set to a potential higher than VPI+V_(th), from the time T4 to time T5, a signal based on imaging data is output to the output terminal (OUT) in accordance with the potential of the node FD2 (selection operation). Through the operation from the time T1 to the time T5, the first operation can be performed.

Next, second operation of the circuit in FIG. 1 is described with reference to a timing chart in FIG. 41.

From the time T1 to the time T2, the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to a potential higher than VPR+V_(th), and the wiring 27 (W) is set to a potential higher than V_(th). At this time, the potential of the node FD1 is set to the potential of the wiring 22 (VPR) and the potential of the node FD2 is set to the potential of the wiring 21 (VPD), i.e., “GND.”

From the time T2 to the time T3, the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential higher than V_(th). Here, the potential of the node FD1 is decreased in response to light with which the photodiode 60 is irradiated. When the amount of decrease in the potential of the node FD1 at the time T3 is denoted by V1, the potential of the node FD1 is VPR−V1. Note that in the circuit structure in FIG. 1, as the intensity of light delivered to the photodiode 60 becomes higher, the potential of the node FD1 becomes lower.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential higher than V_(th) from the time T3 to the time T4, the potential of the node FD1 is held.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential lower than −VPR from the time T4 to the time T5, the potentials of the node FD1 and the node FD2 are held.

In the case where the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to a potential higher than VPR+V_(th), and the wiring 27 (W) is set to a potential lower than −VPR from the time T5 to time T6, the potential of the node FD1 is increased by V1 and the potential of the node FD2 is increased by V2 due to capacitive coupling. Here, V1 and V2 are potentials that reflect illuminance of an initial frame.

In the case where the wiring 25 (TX) is set to a potential higher than VPR+V_(th), the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential lower than −VPR from the time T6 to time T7, the potentials of the node FD1 and the node FD2 are decreased in response to light with which the photodiode 60 is irradiated. When the amount of decrease in the potential of the node FD1 at the time T6 is denoted by V1′, the potential of the node FD1 is VPR−V1′. In addition, the potential of the node FD2 is decreased by V2′ due to capacitive coupling and becomes VPD+V2−V2′.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) is set to a potential lower than −VPR from the time T7 to time T8, the potentials of the node FD1 and the node FD2 are held.

In the case where the wiring 30 (SE) is set to a potential higher than VPI+V_(th) from the time T8 to time T9, a signal based on imaging data is output to the output terminal (OUT) in accordance with the potential of the node FD2. In the above case, the potential of the node FD2 at the time of signal output is GND+V2−V2′; thus, the potential is V2−V2′ when GND is 0 V, for example. Here, V2 is a potential that reflects the illuminance of the initial frame, and V2′ is a potential that reflects illuminance of a later frame (current frame). In other words, the second operation in which a difference between the initial frame and the current frame is output can be performed.

This embodiment can be combined with any of the structures described in the other embodiments or Example as appropriate.

Embodiment 3

In this embodiment, an example of a method for driving a pixel circuit is described.

As described in Embodiment 2, the operation of the pixel circuit is repetition of the reset operation, the accumulation operation, and the selection operation. As imaging modes in which the whole pixel matrix is controlled, a global shutter system and a rolling shutter system are known.

FIG. 13A is a timing chart of a global shutter system. The timing chart illustrates operation of an imaging device in which a plurality of pixel circuits in FIG. 1 are arranged in a matrix. Specifically, the timing chart illustrates operation of the pixel circuits from a first row to an n-th row (n is a natural number of 3 or more). The operation is described giving the first operation described in Embodiment 2 as an example.

In FIG. 13A, a signal 501, a signal 502, and a signal 503 are input to the wirings 26 (PR) connected to the pixel circuits in the first row, the second row, and the n-th row, respectively. A signal 504, a signal 506, and a signal 508 are input to the wirings 25 (TX) connected to the pixel circuits in the first row, the second row, and the n-th row, respectively. A signal 505, a signal 507, and a signal 509 are input to the wirings 29 (PSW) connected to the pixel circuits in the first row, the second row, and the n-th row, respectively.

A period 510 is a period required for one imaging. A period 511 and a period 520 are periods in which reset operation and accumulation operation are performed at the same time in the pixel circuits in each row, respectively. Note that the selection operation is sequentially performed in the pixel circuits in each row. For example, in a period 531, the selection operation is performed in the pixel circuits in the first row. As described above, in the global shutter system, the reset operation and the accumulation operation are performed in all the pixel circuits substantially at the same time, and then read operation is sequentially performed in each row.

That is, in the global shutter system, since the accumulation operation is performed in all the pixel circuits substantially at the same time, imaging is simultaneously performed in the pixel circuits in all the rows. Therefore, an image with little distortion can be obtained even in the case of a moving object.

FIG. 13B is a timing chart of the case using a rolling shutter system. The description of FIG. 13A can be referred to for the signals 501 to 509. A period 610 is a period required for one imaging. A period 611 is a period in which the pixels in the first row perform reset operation. A period 612 is a period in which the pixels in the second row perform reset operation. A period 613 is a period in which the pixels in the n-th row perform reset operation. A period 621 is a period in which the pixels in the first row perform accumulation operation. A period 622 is a period in which the pixels in the second row perform accumulation operation. A period 623 is a period in which the pixels in the n-th row perform accumulation operation. A period 631 is a period in which the pixels in the first row perform selection operation. As described above, in the rolling shutter system, the accumulation operation is not performed at the same time in all the pixel circuits but is sequentially performed in all the rows; thus, imaging is not simultaneously performed in the pixel circuits in all the rows. Therefore, the timing of imaging in the first row is different from that of imaging in the last row, and thus an image with large distortion is obtained in the case of a moving object.

To achieve the global shutter system, the potential of a charge accumulation portion (the node FD2) needs to be held for a long time until sequential reading of signals from the pixels is terminated. When a transistor including a channel formation region formed using an oxide semiconductor and having extremely low off-state current is used as the transistor 55 or the like, the potential of charge accumulation portion (the node FD2) can be held for a long time. In the case where a transistor including a channel formation region formed using silicon or the like is used as the transistor 55 or the like, the potential of the charge accumulation portion (the node FD2) cannot be held for a long time because of high off-state current, which makes it difficult to use the global shutter system.

As described above, the use of the transistor in which a channel formation region is formed using an oxide semiconductor for the pixel circuits makes it easy to achieve the global shutter system.

This embodiment can be combined with any of the structures described in the other embodiments or Example as appropriate.

Embodiment 4

In this embodiment, a transistor including an oxide semiconductor that can be used in one embodiment of the present invention is described with reference to drawings. In the drawings in this embodiment, some components are enlarged, reduced in size, or omitted for easy understanding.

FIGS. 14A and 14B are a top view and a cross-sectional view illustrating a transistor 101 in one embodiment of the present invention. A cross section in the direction of dashed-dotted line B1-B2 in FIG. 14A is illustrated in FIG. 14B. A cross section in the direction of dashed-dotted line B3-B4 in FIG. 14A is illustrated in FIG. 20A. In some cases, the direction of dashed-dotted line B1-B2 is referred to as a channel length direction, and the direction of dashed-dotted line B3-B4 is referred to as a channel width direction.

The transistor 101 includes an insulating layer 120 in contact with a substrate 115; an oxide semiconductor layer 130 in contact with the insulating layer 120; conductive layers 140 and 150 electrically connected to the oxide semiconductor layer 130; an insulating layer 160 in contact with the oxide semiconductor layer 130 and the conductive layers 140 and 150; a conductive layer 170 in contact with the insulating layer 160; an insulating layer 175 in contact with the conductive layers 140 and 150, the insulating layer 160, and the conductive layer 170; and an insulating layer 180 in contact with the insulating layer 175. The insulating layer 180 may function as a planarization film as necessary.

Here, the conductive layer 140, the conductive layer 150, the insulating layer 160, and the conductive layer 170 can function as a source electrode layer, a drain electrode layer, a gate insulating film, and a gate electrode layer, respectively.

A region 231, a region 232, and a region 233 in FIG. 14B can function as a source region, a drain region, and a channel formation region, respectively. The region 231 and the region 232 are in contact with the conductive layer 140 and the conductive layer 150, respectively. When a conductive material that is easily bonded to oxygen is used for the conductive layers 140 and 150, for example, the resistance of the regions 231 and 232 can be reduced.

Specifically, since the oxide semiconductor layer 130 is in contact with the conductive layers 140 and 150, an oxygen vacancy is generated in the oxide semiconductor layer 130, and interaction between the oxygen vacancy and hydrogen that remains in the oxide semiconductor layer 130 or diffuses into the oxide semiconductor layer 130 from the outside changes the regions 231 and 232 to n-type regions with low resistance.

Note that functions of a “source” and a “drain” of a transistor are sometimes interchanged with each other when a transistor of an opposite conductivity type is used or when the direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be interchanged with each other in this specification. In addition, the term “electrode layer” can be changed into the term “wiring.”

The conductive layer 170 includes two layers, conductive layers 171 and 172, but also may be a single layer or a stack of three or more layers. The same applies to other transistors described in this embodiment.

Each of the conductive layers 140 and 150 is a single layer, but also may be a stack of two or more layers. The same applies to other transistors described in this embodiment.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 15A and 15B. FIG. 15A is a top view of a transistor 102. A cross section in the direction of dashed-dotted line C1-C2 in FIG. 15A is illustrated in FIG. 15B. A cross section in the direction of dashed-dotted line C3-C4 in FIG. 15A is illustrated in FIG. 20B. In some cases, the direction of dashed-dotted line C1-C2 is referred to as a channel length direction, and the direction of dashed-dotted line C3-C4 is referred to as a channel width direction.

The transistor 102 has the same structure as the transistor 101 except that an end portion of the insulating layer 160 functioning as a gate insulating film is not aligned with an end portion of the conductive layer 170 functioning as a gate electrode layer. In the transistor 102, wide areas of the conductive layers 140 and 150 are covered with the insulating layer 160 and accordingly the resistance between the conductive layer 170 and the conductive layers 140 and 150 is high; therefore, the transistor 102 has low gate leakage current.

The transistors 101 and 102 each have a top-gate structure including a region where the conductive layer 170 overlaps with the conductive layers 140 and 150. To reduce parasitic capacitance, the width of the region in the channel length direction is preferably greater than or equal to 3 nm and less than 300 nm. Since an offset region is not formed in the oxide semiconductor layer 130 in this structure, a transistor with high on-state current can be easily formed.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 16A and 16B. FIG. 16A is a top view of a transistor 103. A cross section in the direction of dashed-dotted line D1-D2 in FIG. 16A is illustrated in FIG. 16B. A cross section in the direction of dashed-dotted line D3-D4 in FIG. 16A is illustrated in FIG. 20A. In some cases, the direction of dashed-dotted line D1-D2 is referred to as a channel length direction, and the direction of dashed-dotted line D3-D4 is referred to as a channel width direction.

The transistor 103 includes the insulating layer 120 in contact with the substrate 115; the oxide semiconductor layer 130 in contact with the insulating layer 120; the insulating layer 160 in contact with the oxide semiconductor layer 130; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 covering the oxide semiconductor layer 130, the insulating layer 160, and the conductive layer 170; the insulating layer 180 in contact with the insulating layer 175; and the conductive layers 140 and 150 electrically connected to the oxide semiconductor layer 130 through openings provided in the insulating layers 175 and 180. The transistor 103 may further include, for example, an insulating layer (planarization film) in contact with the insulating layer 180 and the conductive layers 140 and 150 as necessary.

Here, the conductive layer 140, the conductive layer 150, the insulating layer 160, and the conductive layer 170 can function as a source electrode layer, a drain electrode layer, a gate insulating film, and a gate electrode layer, respectively.

The region 231, the region 232, and the region 233 in FIG. 16B can function as a source region, a drain region, and a channel formation region, respectively. The regions 231 and 232 are in contact with the insulating layer 175. When an insulating material containing hydrogen is used for the insulating layer 175, for example, the resistance of the regions 231 and 232 can be reduced.

Specifically, interaction between an oxygen vacancy generated in the regions 231 and 232 by the steps up to formation of the insulating layer 175 and hydrogen that diffuses into the regions 231 and 232 from the insulating layer 175 changes the regions 231 and 232 to n-type regions with low resistance. As the insulating material containing hydrogen, for example, silicon nitride, aluminum nitride, or the like can be used.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 17A and 17B. FIG. 17A is a top view of a transistor 104. A cross section in the direction of dashed-dotted line E1-E2 in FIG. 17A is illustrated in FIG. 17B. A cross section in the direction of dashed-dotted line E3-E4 in FIG. 17A is illustrated in FIG. 20A. In some cases, the direction of dashed-dotted line E1-E2 is referred to as a channel length direction, and the direction of dashed-dotted line E3-E4 is referred to as a channel width direction.

The transistor 104 has the same structure as the transistor 103 except that the conductive layers 140 and 150 in contact with the oxide semiconductor layer 130 cover end portions of the oxide semiconductor layer 130.

In FIG. 17B, regions 331 and 334 can function as a source region, regions 332 and 335 can function as a drain region, and a region 333 can function as a channel formation region.

The resistance of the regions 331 and 332 can be reduced in a manner similar to that of the regions 231 and 232 in the transistor 101.

The resistance of the regions 334 and 335 can be reduced in a manner similar to that of the regions 231 and 232 in the transistor 103. In the case where the length of the regions 334 and 335 in the channel length direction is less than or equal to 100 nm, preferably less than or equal to 50 nm, a gate electric field prevents a significant decrease in on-state current. Therefore, a reduction in resistance of the regions 334 and 335 is not performed in some cases.

The transistors 103 and 104 each have a self-aligned structure that does not include a region where the conductive layer 170 overlaps with the conductive layers 140 and 150. A transistor with a self-aligned structure, which has extremely low parasitic capacitance between a gate electrode layer and source and drain electrode layers, is suitable for applications that require high-speed operation.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 18A and 18B. FIG. 18A is a top view of a transistor 105. A cross section in the direction of dashed-dotted line F1-F2 in FIG. 18A is illustrated in FIG. 18B. A cross section in the direction of dashed-dotted line F3-F4 in FIG. 18A is illustrated in FIG. 20A. In some cases, the direction of dashed-dotted line F1-F2 is referred to as a channel length direction, and the direction of dashed-dotted line F3-F4 is referred to as a channel width direction.

The transistor 105 includes the insulating layer 120 in contact with the substrate 115; the oxide semiconductor layer 130 in contact with the insulating layer 120; conductive layers 141 and 151 electrically connected to the oxide semiconductor layer 130; the insulating layer 160 in contact with the oxide semiconductor layer 130 and the conductive layers 141 and 151; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 in contact with the oxide semiconductor layer 130, the conductive layers 141 and 151, the insulating layer 160, and the conductive layer 170; the insulating layer 180 in contact with the insulating layer 175; and conductive layers 142 and 152 electrically connected to the conductive layers 141 and 151, respectively, through openings provided in the insulating layers 175 and 180. The transistor 105 may further include, for example, an insulating layer in contact with the insulating layer 180 and the conductive layers 142 and 152 as necessary.

Here, the conductive layers 141 and 151 are in contact with the top surface of the oxide semiconductor layer 130 and are not in contact with side surfaces of the oxide semiconductor layer 130.

The transistor 105 has the same structure as the transistor 101 except that the conductive layers 141 and 151 are provided, that openings are provided in the insulating layers 175 and 180, and that the conductive layers 142 and 152 electrically connected to the conductive layers 141 and 151, respectively, through the openings are provided. The conductive layer 140 (the conductive layers 141 and 142) can function as a source electrode layer, and the conductive layer 150 (the conductive layers 151 and 152) can function as a drain electrode layer.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 19A and 19B. FIG. 19A is a top view of a transistor 106. A cross section in the direction of dashed-dotted line G1-G2 in FIG. 19A is illustrated in FIG. 19B. A cross section in the direction of dashed-dotted line G3-G4 in FIG. 19A is illustrated in FIG. 20A. In some cases, the direction of dashed-dotted line G1-G2 is referred to as a channel length direction, and the direction of dashed-dotted line G3-G4 is referred to as a channel width direction.

The transistor 106 includes the insulating layer 120 in contact with the substrate 115; the oxide semiconductor layer 130 in contact with the insulating layer 120; the conductive layers 141 and 151 electrically connected to the oxide semiconductor layer 130; the insulating layer 160 in contact with the oxide semiconductor layer 130; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 in contact with the insulating layer 120, the oxide semiconductor layer 130, the conductive layers 141 and 151, the insulating layer 160, and the conductive layer 170; the insulating layer 180 in contact with the insulating layer 175; and the conductive layers 142 and 152 electrically connected to the conductive layers 141 and 151, respectively, through openings provided in the insulating layers 175 and 180. The transistor 106 may further include, for example, an insulating layer (planarization film) in contact with the insulating layer 180 and the conductive layers 142 and 152 as necessary.

Here, the conductive layers 141 and 151 are in contact with the top surface of the oxide semiconductor layer 130 and are not in contact with side surfaces of the oxide semiconductor layer 130.

The transistor 106 has the same structure as the transistor 103 except that the conductive layers 141 and 151 are provided. The conductive layer 140 (the conductive layers 141 and 142) can function as a source electrode layer, and the conductive layer 150 (the conductive layers 151 and 152) can function as a drain electrode layer.

In the structures of the transistors 105 and 106, the conductive layers 140 and 150 are not in contact with the insulating layer 120. These structures make the insulating layer 120 less likely to be deprived of oxygen by the conductive layers 140 and 150 and facilitate oxygen supply from the insulating layer 120 to the oxide semiconductor layer 130.

An impurity for forming an oxygen vacancy to increase conductivity may be added to the regions 231 and 232 in the transistor 103 and the regions 334 and 335 in the transistors 104 and 106. As an impurity for forming an oxygen vacancy in an oxide semiconductor layer, for example, one or more of the following can be used: phosphorus, arsenic, antimony, boron, aluminum, silicon, nitrogen, helium, neon, argon, krypton, xenon, indium, fluorine, chlorine, titanium, zinc, and carbon. As a method for adding the impurity, plasma treatment, ion implantation, ion doping, plasma immersion ion implantation, or the like can be used.

When the above element is added as an impurity element to the oxide semiconductor layer, a bond between a metal element and oxygen in the oxide semiconductor layer is cut, so that an oxygen vacancy is formed. Interaction between an oxygen vacancy in the oxide semiconductor layer and hydrogen that remains in the oxide semiconductor layer or is added to the oxide semiconductor layer later can increase the conductivity of the oxide semiconductor layer.

When hydrogen is added to an oxide semiconductor in which an oxygen vacancy is formed by addition of an impurity element, hydrogen enters an oxygen vacant site and forms a donor level in the vicinity of the conduction band. Consequently, an oxide conductor can be formed. Here, an oxide conductor refers to an oxide semiconductor having become a conductor. Note that the oxide conductor has a light-transmitting property in a manner similar to the oxide semiconductor.

The oxide conductor is a degenerated semiconductor and it is suggested that the conduction band edge equals or substantially equals the Fermi level. For that reason, an ohmic contact is made between an oxide conductor layer and conductive layers functioning as a source electrode layer and a drain electrode layer; thus, contact resistance between the oxide conductor layer and the conductive layers functioning as a source electrode layer and a drain electrode layer can be reduced.

The transistor in one embodiment of the present invention may include a conductive layer 173 between the oxide semiconductor layer 130 and the substrate 115 as illustrated in cross-sectional views in the channel length direction in FIGS. 21A to 21F and cross-sectional views in the channel width direction in FIGS. 20C and 20D. When the conductive layer 173 is used as a second gate electrode layer (back gate), the on-state current can be increased or the threshold voltage can be controlled. In the cross-sectional views in FIGS. 21A to 21F, the width of the conductive layer 173 may be shorter than that of the oxide semiconductor layer 130. Moreover, the width of the conductive layer 173 may be shorter than that of the conductive layer 170.

In order to increase the on-state current, for example, the conductive layers 170 and 173 are made to have the same potential, and the transistor is driven as a double-gate transistor. Furthermore, in order to control the threshold voltage, a fixed potential that is different from the potential of the conductive layer 170 is applied to the conductive layer 173. To set the conductive layers 170 and 173 at the same potential, for example, as illustrated in FIG. 20D, the conductive layers 170 and 173 may be electrically connected to each other through a contact hole.

Although the transistors 101 to 106 in FIGS. 14A and 14B, FIGS. 15A and 15B, FIGS. 16A and 16B, FIGS. 17A and 17B, FIGS. 18A and 18B, and FIGS. 19A and 19B are examples in which the oxide semiconductor layer 130 is a single layer, the oxide semiconductor layer 130 may be a stacked layer. The oxide semiconductor layer 130 in the transistors 101 to 106 can be replaced with the oxide semiconductor layer 130 in FIGS. 22A to 22C or FIGS. 23A to 23C.

FIGS. 22A to 22C are a top view and cross-sectional views of the oxide semiconductor layer 130 with a two-layer structure. FIG. 22B illustrates a cross section in the direction of dashed-dotted line A1-A2 in FIG. 22A. FIG. 22C illustrates a cross section in the direction of dashed-dotted line A3-A4 in FIG. 22A.

FIGS. 23A to 23C are a top view and cross-sectional views of the oxide semiconductor layer 130 with a three-layer structure. FIG. 23B illustrates a cross section in the direction of dashed-dotted line A1-A2 in FIG. 23A. FIG. 23C illustrates a cross section in the direction of dashed-dotted line A3-A4 in FIG. 23A.

Oxide semiconductor layers with different compositions, for example, can be used as an oxide semiconductor layer 130 a, an oxide semiconductor layer 130 b, and an oxide semiconductor layer 130 c.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 24A and 24B. FIG. 24A is a top view of a transistor 107. A cross section in the direction of dashed-dotted line H1-H2 in FIG. 24A is illustrated in FIG. 24B. A cross section in the direction of dashed-dotted line H3-H4 in FIG. 24A is illustrated in FIG. 30A. In some cases, the direction of dashed-dotted line H1-H2 is referred to as a channel length direction, and the direction of dashed-dotted line H3-H4 is referred to as a channel width direction.

The transistor 107 includes the insulating layer 120 in contact with the substrate 115; a stack of the oxide semiconductor layers 130 a and 130 b in contact with the insulating layer 120; the conductive layers 140 and 150 electrically connected to the stack; the oxide semiconductor layer 130 c in contact with the stack and the conductive layers 140 and 150; the insulating layer 160 in contact with the oxide semiconductor layer 130 c; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 in contact with the conductive layers 140 and 150, the oxide semiconductor layer 130 c, the insulating layer 160, and the conductive layer 170; and the insulating layer 180 in contact with the insulating layer 175. The insulating layer 180 may function as a planarization film as necessary.

The transistor 107 has the same structure as the transistor 101 except that the oxide semiconductor layer 130 includes two layers (the oxide semiconductor layers 130 a and 130 b) in the regions 231 and 232, that the oxide semiconductor layer 130 includes three layers (the oxide semiconductor layers 130 a to 130 c) in the region 233, and that part of the oxide semiconductor layer (the oxide semiconductor layer 130 c) exists between the insulating layer 160 and the conductive layers 140 and 150.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 25A and 25B. FIG. 25A is a top view of a transistor 108. A cross section in the direction of dashed-dotted line I1-12 in FIG. 25A is illustrated in FIG. 25B. A cross section in the direction of dashed-dotted line 13-14 in FIG. 25A is illustrated in FIG. 30B. In some cases, the direction of dashed-dotted line I1-I2 is referred to as a channel length direction, and the direction of dashed-dotted line 13-14 is referred to as a channel width direction.

The transistor 108 differs from the transistor 107 in that end portions of the insulating layer 160 and the oxide semiconductor layer 130 c are not aligned with the end portion of the conductive layer 170.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 26A and 26B. FIG. 26A is a top view of a transistor 109. A cross section in the direction of dashed-dotted line J1-J2 in FIG. 26A is illustrated in FIG. 26B. A cross section in the direction of dashed-dotted line J3-J4 in FIG. 26A is illustrated in FIG. 30A. In some cases, the direction of dashed-dotted line J1-J2 is referred to as a channel length direction, and the direction of dashed-dotted line J3-J4 is referred to as a channel width direction.

The transistor 109 includes the insulating layer 120 in contact with the substrate 115; a stack of the oxide semiconductor layers 130 a and 130 b in contact with the insulating layer 120; the oxide semiconductor layer 130 c in contact with the stack; the insulating layer 160 in contact with the oxide semiconductor layer 130 c; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 covering the stack, the oxide semiconductor layer 130 c, the insulating layer 160, and the conductive layer 170; the insulating layer 180 in contact with the insulating layer 175; and the conductive layers 140 and 150 electrically connected to the stack through openings provided in the insulating layers 175 and 180. The transistor 109 may further include, for example, an insulating layer (planarization film) in contact with the insulating layer 180 and the conductive layers 140 and 150 as necessary.

The transistor 109 has the same structure as the transistor 103 except that the oxide semiconductor layer 130 includes two layers (the oxide semiconductor layers 130 a and 130 b) in the regions 231 and 232 and that the oxide semiconductor layer 130 includes three layers (the oxide semiconductor layers 130 a to 130 c) in the region 233.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 27A and 27B. FIG. 27A is a top view of a transistor 110. A cross section in the direction of dashed-dotted line K1-K2 in FIG. 27A is illustrated in FIG. 27B. A cross section in the direction of dashed-dotted line K3-K4 in FIG. 27A is illustrated in FIG. 30A. In some cases, the direction of dashed-dotted line K1-K2 is referred to as a channel length direction, and the direction of dashed-dotted line K3-K4 is referred to as a channel width direction.

The transistor 110 has the same structure as the transistor 104 except that the oxide semiconductor layer 130 includes two layers (the oxide semiconductor layers 130 a and 130 b) in the regions 231 and 232 and that the oxide semiconductor layer 130 includes three layers (the oxide semiconductor layers 130 a to 130 c) in the region 233.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 28A and 28B. FIG. 28A is a top view of a transistor 111. A cross section in the direction of dashed-dotted line L1-L2 in FIG. 28A is illustrated in FIG. 28B. A cross section in the direction of dashed-dotted line L3-L4 in FIG. 28A is illustrated in FIG. 30A. In some cases, the direction of dashed-dotted line L1-L2 is referred to as a channel length direction, and the direction of dashed-dotted line L3-L4 is referred to as a channel width direction.

The transistor 111 includes the insulating layer 120 in contact with the substrate 115; a stack of the oxide semiconductor layers 130 a and 130 b in contact with the insulating layer 120; the conductive layers 141 and 151 electrically connected to the stack; the oxide semiconductor layer 130 c in contact with the stack and the conductive layers 141 and 151; the insulating layer 160 in contact with the oxide semiconductor layer 130 c; the conductive layer 170 in contact with the insulating layer 160; the insulating layer 175 in contact with the stack, the conductive layers 141 and 151, the oxide semiconductor layer 130 c, the insulating layer 160, and the conductive layer 170; the insulating layer 180 in contact with the insulating layer 175; and the conductive layers 142 and 152 electrically connected to the conductive layers 141 and 151, respectively, through openings provided in the insulating layers 175 and 180. The transistor 111 may further include, for example, an insulating layer (planarization film) in contact with the insulating layer 180 and the conductive layers 142 and 152 as necessary.

The transistor 111 has the same structure as the transistor 105 except that the oxide semiconductor layer 130 includes two layers (the oxide semiconductor layers 130 a and 130 b) in the regions 231 and 232, that the oxide semiconductor layer 130 includes three layers (the oxide semiconductor layers 130 a to 130 c) in the region 233, and that part of the oxide semiconductor layer (the oxide semiconductor layer 130 c) exists between the insulating layer 160 and the conductive layers 141 and 151.

The transistor in one embodiment of the present invention may have a structure illustrated in FIGS. 29A and 29B. FIG. 29A is a top view of a transistor 112. A cross section in the direction of dashed-dotted line M1-M2 in FIG. 29A is illustrated in FIG. 29B. A cross section in the direction of dashed-dotted line M3-M4 in FIG. 29A is illustrated in FIG. 30A. In some cases, the direction of dashed-dotted line M1-M2 is referred to as a channel length direction, and the direction of dashed-dotted line M3-M4 is referred to as a channel width direction.

The transistor 112 has the same structure as the transistor 106 except that the oxide semiconductor layer 130 includes two layers (the oxide semiconductor layers 130 a and 130 b) in the regions 331, 332, 334, and 335 and that the oxide semiconductor layer 130 includes three layers (the oxide semiconductor layers 130 a to 130 c) in the region 333.

The transistor in one embodiment of the present invention may include the conductive layer 173 between the oxide semiconductor layer 130 and the substrate 115 as illustrated in cross-sectional views in the channel length direction in FIGS. 31A to 31F and cross-sectional views in the channel width direction in FIGS. 30C and 30D. When the conductive layer is used as a second gate electrode layer (back gate), the on-state current can be increased or the threshold voltage can be controlled. In the cross-sectional views in FIGS. 31A to 31F, the width of the conductive layer 173 may be shorter than that of the oxide semiconductor layer 130. Moreover, the width of the conductive layer 173 may be shorter than that of the conductive layer 170.

As illustrated in FIG. 32A, the width (W_(SD)) of the conductive layer 140 (source electrode layer) and the conductive layer 150 (drain electrode layer) in the transistor in one embodiment of the present invention may be longer than the width (W_(OS)) of the oxide semiconductor layer. Furthermore, as illustrated in FIG. 32B, W_(SD) may be shorter than W_(OS). When W_(OS)≧W_(SD) (W_(SD) is less than or equal to W_(OS)) is satisfied, a gate electric field is easily applied to the entire oxide semiconductor layer 130, so that electrical characteristics of the transistor can be improved.

In the transistor in one embodiment of the present invention (any of the transistors 101 to 112), the conductive layer 170 functioning as a gate electrode layer electrically surrounds the oxide semiconductor layer 130 in the channel width direction with the insulating layer 160 functioning as a gate insulating film positioned therebetween. This structure increases the on-state current. Such a transistor structure is referred to as a surrounded channel (s-channel) structure.

In the transistor including the oxide semiconductor layers 130 a and 130 b and the transistor including the oxide semiconductor layers 130 a to 130 c, selecting appropriate materials for the two or three layers forming the oxide semiconductor layer 130 makes current flow to the oxide semiconductor layer 130 b. Since current flows to the oxide semiconductor layer 130 b, the current is hardly influenced by interface scattering, leading to high on-state current. Note that increasing the thickness of the oxide semiconductor layer 130 b can increase the on-state current. The thickness of the oxide semiconductor layer 130 b may be, for example, 100 to 200 nm.

A semiconductor device including a transistor with any of the above structures can have favorable electrical characteristics.

Note that in this specification, the channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or in a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, the channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

Note that depending on transistor structures, a channel width in a region where a channel is actually formed (hereinafter referred to as an effective channel width) is sometimes different from a channel width shown in a top view of a transistor (hereinafter referred to as an apparent channel width). For example, in a transistor having a gate electrode covering a side surface of a semiconductor, an effective channel width is greater than an apparent channel width, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a gate electrode covering a side surface of a semiconductor, the proportion of a channel region formed in a side surface of a semiconductor is increased. In that case, an effective channel width is greater than an apparent channel width.

In such a case, measuring an effective channel width is difficult in some cases. For example, to estimate an effective channel width from a design value, it is necessary to assume that the shape of a semiconductor is known. Therefore, in the case where the shape of a semiconductor is not known accurately, measuring an effective channel width accurately is difficult.

Accordingly, in this specification, an apparent channel width is referred to as a surrounded channel width (SCW) in some cases. Furthermore, in this specification, the term “channel width” may denote a surrounded channel width, i.e., an apparent channel width or an effective channel width. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional TEM image and the like.

A surrounded channel width may be used to calculate field-effect mobility, a current value per channel width, and the like of a transistor. In this case, the obtained value is sometimes different from the value obtained by using an effective channel width for the calculation.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments and Example as appropriate.

Embodiment 5

In this embodiment, components of the transistors described in Embodiment 4 are described in detail.

The substrate 115 includes a silicon substrate provided with a transistor and/or a photodiode; and an insulating layer, a wiring, a conductor functioning as a contact plug, and the like that are provided over the silicon substrate. Note that when p-ch transistors are formed using the silicon substrate, a silicon substrate with n⁻-type conductivity is preferably used. Alternatively, an SOI substrate including an n⁻-type or i-type silicon layer may be used. A surface of the silicon substrate where the transistor is formed preferably has a (110) plane orientation. Forming a p-ch transistor with the (110) plane can increase mobility.

The insulating layer 120 can have a function of supplying oxygen to the oxide semiconductor layer 130 as well as a function of preventing diffusion of impurities from a component included in the substrate 115. For this reason, the insulating layer 120 is preferably an insulating film containing oxygen and more preferably, the insulating layer 120 is an insulating film containing oxygen in which the oxygen content is higher than that in the stoichiometric composition. The insulating layer 120 is a film in which the amount of released oxygen when converted into oxygen atoms is preferably greater than or equal to 1.0×10¹⁹ atoms/cm³ in TDS analysis. In the TDS analysis, the film surface temperature is higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. The insulating layer 120 also functions as an interlayer insulating film and may be subjected to planarization treatment such as chemical mechanical polishing (CMP) so as to have a flat surface.

For example, the insulating layer 120 can be formed using an oxide insulating film including aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like; a nitride insulating film including silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like; or a mixed material of any of these. The insulating layer 120 may be a stack of any of the above materials.

In this embodiment, detailed description is given mainly on the case where the oxide semiconductor layer 130 of the transistor has a three-layer structure in which the oxide semiconductor layers 130 a to 130 c are sequentially stacked from the insulating layer 120 side.

Note that in the case where the oxide semiconductor layer 130 is a single layer, a layer corresponding to the oxide semiconductor layer 130 b described in this embodiment is used.

In the case where the oxide semiconductor layer 130 has a two-layer structure, a stack in which a layer corresponding to the oxide semiconductor layer 130 a and a layer corresponding to the oxide semiconductor layer 130 b are sequentially stacked from the insulating layer 120 side described in this embodiment is used. In such a case, the oxide semiconductor layers 130 a and 130 b can be replaced with each other.

In the case where the oxide semiconductor layer 130 has a layered structure of four or more layers, for example, a structure in which another oxide semiconductor layer is added to the three-layer stack of the oxide semiconductor layer 130 described in this embodiment can be employed.

For the oxide semiconductor layer 130 b, for example, an oxide semiconductor whose electron affinity (an energy difference between a vacuum level and the conduction band minimum) is higher than those of the oxide semiconductor layers 130 a and 130 c is used. The electron affinity can be obtained by subtracting an energy difference between the conduction band minimum and the valence band maximum (what is called an energy gap) from an energy difference between the vacuum level and the valence band maximum (what is called an ionization potential).

The oxide semiconductor layers 130 a and 130 c each contain one or more kinds of metal elements contained in the oxide semiconductor layer 130 b. For example, the oxide semiconductor layers 130 a and 130 c are preferably formed using an oxide semiconductor whose conduction band minimum is closer to a vacuum level than that of the oxide semiconductor layer 130 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

In such a structure, when an electric field is applied to the conductive layer 170, a channel is formed in the oxide semiconductor layer 130 b whose conduction band minimum is the lowest in the oxide semiconductor layer 130.

Furthermore, since the oxide semiconductor layer 130 a contains one or more kinds of metal elements contained in the oxide semiconductor layer 130 b, an interface state is unlikely to be formed at the interface between the oxide semiconductor layers 130 a and 130 b, compared with the interface between the oxide semiconductor layer 130 b and the insulating layer 120 on the assumption that the oxide semiconductor layer 130 b is in contact with the insulating layer 120. The interface state sometimes forms a channel; therefore, the threshold voltage of the transistor is changed in some cases. Thus, with the oxide semiconductor layer 130 a, variations in electrical characteristics of the transistor, such as a threshold voltage, can be reduced. Moreover, the reliability of the transistor can be improved.

Furthermore, since the oxide semiconductor layer 130 c contains one or more kinds of metal elements contained in the oxide semiconductor layer 130 b, scattering of carriers is unlikely to occur at the interface between the oxide semiconductor layers 130 b and 130 c, compared with the interface between the oxide semiconductor layer 130 b and the gate insulating film (the insulating layer 160) on the assumption that the oxide semiconductor layer 130 b is in contact with the gate insulating film. Thus, with the oxide semiconductor layer 130 c, the field-effect mobility of the transistor can be increased.

For the oxide semiconductor layers 130 a and 130 c, for example, a material containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that used for the oxide semiconductor layer 130 b can be used. Specifically, the atomic ratio of any of the above metal elements in the oxide semiconductor layers 130 a and 130 c is 1.5 times or more, preferably 2 times or more, more preferably 3 times or more as large as that in the oxide semiconductor layer 130 b. Any of the above metal elements is strongly bonded to oxygen and thus has a function of suppressing generation of an oxygen vacancy in the oxide semiconductor layers 130 a and 130 c. That is, an oxygen vacancy is less likely to be generated in the oxide semiconductor layers 130 a and 130 c than in the oxide semiconductor layer 130 b.

An oxide semiconductor that can be used for each of the oxide semiconductor layers 130 a to 130 c preferably contains at least In or Zn. Both In and Zn are preferably contained. In order to reduce variations in electrical characteristics of the transistor including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn.

Examples of a stabilizer include Ga, Sn, Hf, Al, and Zr. Other examples of the stabilizer include lanthanoids such as La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, gallium oxide, zinc oxide, an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, an In—Ga—Zn oxide means an oxide containing In, Ga, and Zn as its main components. The In—Ga—Zn oxide may contain another metal element in addition to In, Ga, and Zn. In this specification, a film containing the In—Ga—Zn oxide is also referred to as an IGZO film.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Y, Zr, La, Ce, and Nd. Alternatively, a material represented by InzSnO₅(ZnO)_(n) (n>0, where n is an integer) may be used.

Note that when each of the oxide semiconductor layers 130 a to 130 c is an In-M-Zn oxide containing at least indium, zinc, and M (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), in the case where the oxide semiconductor layer 130 a has an atomic ratio of In to M and Zn which is x₁:y₁:z₁, the oxide semiconductor layer 130 b has an atomic ratio of In to M and Zn which is x₂:y₂:z₂, and the oxide semiconductor layer 130 c has an atomic ratio of In to M and Zn which is x₃:y₃:z₃, each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Each of y₁/x₁ and y₃/x₃ is 1.5 times or more, preferably 2 times or more, more preferably 3 times or more as large as y₂/x₂. At this time, when y₂ is greater than or equal to x₂ in the oxide semiconductor layer 130 b, the transistor can have stable electrical characteristics. However, when y₂ is 3 times or more as large as x₂, the field-effect mobility of the transistor is reduced; accordingly, y₂ is preferably smaller than 3 times x₂.

In the case where Zn and O are not taken into consideration, the proportion of In and the proportion of M in each of the oxide semiconductor layers 130 a and 130 c are preferably less than 50 atomic % and greater than or equal to 50 atomic %, respectively, more preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. Furthermore, in the case where Zn and O are not taken into consideration, the proportion of In and the proportion of M in the oxide semiconductor layer 130 b are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, more preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively.

The indium content in the oxide semiconductor layer 130 b is preferably higher than those in the oxide semiconductor layers 130 a and 130 c. In an oxide semiconductor, the s orbital of heavy metal mainly contributes to carrier transfer, and when the proportion of In in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Therefore, an oxide in which the proportion of In is higher than that of M has higher mobility than an oxide in which the proportion of In is equal to or lower than that of M. Thus, with the use of an oxide having a high content of indium for the oxide semiconductor layer 130 b, a transistor having high field-effect mobility can be obtained.

The thickness of the oxide semiconductor layer 130 a is greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, more preferably greater than or equal to 5 nm and less than or equal to 25 nm. The thickness of the oxide semiconductor layer 130 b is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 10 nm and less than or equal to 150 nm, more preferably greater than or equal to 15 nm and less than or equal to 100 nm. The thickness of the oxide semiconductor layer 130 c is greater than or equal to 1 nm and less than or equal to 50 nm, preferably greater than or equal to 2 nm and less than or equal to 30 nm, more preferably greater than or equal to 3 nm and less than or equal to 15 nm. In addition, the oxide semiconductor layer 130 b is preferably thicker than the oxide semiconductor layers 130 a and 130 c.

Note that in order that a transistor in which a channel is formed in an oxide semiconductor layer have stable electrical characteristics, it is effective to make the oxide semiconductor layer intrinsic or substantially intrinsic by reducing the concentration of impurities in the oxide semiconductor layer. The term “substantially intrinsic” refers to a state where an oxide semiconductor layer has a carrier density lower than 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than 1×10¹³/cm³.

In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon, and a metal element other than main components of the oxide semiconductor layer are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density, and silicon forms impurity levels in the oxide semiconductor layer. The impurity levels serve as traps and might cause deterioration of electrical characteristics of the transistor. Therefore, it is preferable to reduce the concentration of the impurities in the oxide semiconductor layers 130 a to 130 c and at interfaces between the oxide semiconductor layers.

In order to make the oxide semiconductor layer intrinsic or substantially intrinsic, the oxide semiconductor layer is controlled to have a region in which the concentration of silicon estimated by secondary ion mass spectrometry (SIMS) is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³. In addition, the oxide semiconductor layer is controlled to have a region in which the concentration of hydrogen is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹ atoms/cm³, still more preferably lower than or equal to 5×10¹⁸ atoms/cm³. Furthermore, the concentration of nitrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³, still more preferably lower than or equal to 5×10¹⁷ atoms/cm³.

The high concentration of silicon or carbon might reduce the crystallinity of the oxide semiconductor layer. In order not to lower the crystallinity of the oxide semiconductor layer, for example, the oxide semiconductor layer is controlled to have a region in which the concentration of silicon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³. Furthermore, the oxide semiconductor layer is controlled to have a region in which the concentration of carbon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³.

A transistor in which a highly purified oxide semiconductor film is used for a channel formation region exhibits extremely low off-state current. When voltage between a source and a drain is set at about 0.1 V, 5 V, or 10 V, for example, the off-state current per channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer.

As the gate insulating film of the transistor, an insulating film containing silicon is used in many cases; thus, it is preferable that, as in the transistor in one embodiment of the present invention, a region of the oxide semiconductor layer that serves as a channel not be in contact with the gate insulating film for the above reason. In the case where a channel is formed at the interface between the gate insulating film and the oxide semiconductor layer, scattering of carriers occurs at the interface, so that the field-effect mobility of the transistor is reduced. Also from the view of the above, it is preferable that the region of the oxide semiconductor layer that serves as a channel be separated from the gate insulating film.

Accordingly, with the oxide semiconductor layer 130 having a layered structure including the oxide semiconductor layers 130 a to 130 c, a channel can be formed in the oxide semiconductor layer 130 b; thus, the transistor can have high field-effect mobility and stable electrical characteristics.

In a band structure, the conduction band minimums of the oxide semiconductor layers 130 a to 130 c are continuous. This can be understood also from the fact that the compositions of the oxide semiconductor layers 130 a to 130 c are close to one another and oxygen is easily diffused among the oxide semiconductor layers 130 a to 130 c. Thus, the oxide semiconductor layers 130 a to 130 c have a continuous physical property though they have different compositions and form a stack. In the drawings, interfaces between the oxide semiconductor layers of the stack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the same main components are stacked is formed to have not only a simple layered structure of the layers but also a continuous energy band (here, in particular, a well structure having a U shape in which the conduction band minimums are continuous (U-shape well)). In other words, the layered structure is formed such that there exists no impurity that forms a defect level such as a trap center or a recombination center at each interface. If impurities exist between the stacked oxide semiconductor layers, the continuity of the energy band is lost and carriers disappear by a trap or recombination at the interface.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6 can be used for the oxide semiconductor layers 130 a and 130 c, and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1, 2:1:3, 5:5:6, or 3:1:2 can be used for the oxide semiconductor layer 130 b. In each of the oxide semiconductor layers 130 a to 130 c, the proportion of each atom in the atomic ratio varies within a range of ±20% as a margin.

The oxide semiconductor layer 130 b of the oxide semiconductor layer 130 serves as a well, so that a channel is formed in the oxide semiconductor layer 130 b. Note that since the conduction band minimums are continuous, the oxide semiconductor layer 130 can also be referred to as a U-shaped well. Furthermore, a channel formed to have such a structure can also be referred to as a buried channel.

Note that trap levels due to impurities or defects might be formed in the vicinity of the interface between an insulating layer such as a silicon oxide film and each of the oxide semiconductor layers 130 a and 130 c. The oxide semiconductor layer 130 b can be distanced away from the trap levels owing to existence of the oxide semiconductor layers 130 a and 130 c.

However, when the energy differences between the conduction band minimum of the oxide semiconductor layer 130 b and the conduction band minimum of each of the oxide semiconductor layers 130 a and 130 c are small, an electron in the oxide semiconductor layer 130 b might reach the trap level by passing over the energy differences. When the electron is trapped in the trap level, negative charge is generated at the interface with the insulating layer, so that the threshold voltage of the transistor is shifted in a positive direction.

The oxide semiconductor layers 130 a to 130 c preferably include crystal parts. In particular, when crystals with c-axis alignment are used, the transistor can have stable electrical characteristics. Moreover, crystals with c-axis alignment are resistant to bending; therefore, using such crystals can improve the reliability of a semiconductor device using a flexible substrate.

As the conductive layer 140 functioning as a source electrode layer and the conductive layer 150 functioning as a drain electrode layer, for example, a single layer or a stacked layer formed using a material selected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloys of any of these metal materials can be used. Typically, it is preferable to use Ti, which is particularly easily bonded to oxygen, or W, which has a high melting point and thus makes subsequent process temperatures comparatively high. It is also possible to use a stack of any of the above materials and Cu or an alloy such as Cu—Mn, which has low resistance. In the transistors 105, 106, 111, and 112, for example, it is possible to use W for the conductive layers 141 and 151 and use a stack of Ti and Al for the conductive layers 142 and 152.

The above materials are capable of extracting oxygen from an oxide semiconductor layer. Therefore, in a region of the oxide semiconductor layer that is in contact with any of the above materials, oxygen is released from the oxide semiconductor layer and an oxygen vacancy is formed. Hydrogen slightly contained in the layer and the oxygen vacancy are bonded to each other, so that the region is changed to an n-type region. Accordingly, the n-type region can serve as a source or a drain of the transistor.

In the case where W is used for the conductive layers 140 and 150, the conductive layers 140 and 150 may be doped with nitrogen. Doping with nitrogen can appropriately lower the capability of extracting oxygen and prevent the n-type region from spreading to a channel region. It is possible to prevent the n-type region from spreading to a channel region also by using a stack of W and an n-type semiconductor layer as the conductive layers 140 and 150 and putting the n-type semiconductor layer in contact with the oxide semiconductor layer. As the n-type semiconductor layer, an In—Ga—Zn oxide, zinc oxide, indium oxide, tin oxide, indium tin oxide, or the like to which nitrogen is added can be used.

The insulating layer 160 functioning as a gate insulating film can be formed using an insulating film containing one or more of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.

The insulating layer 160 may be a stack including any of the above materials. The insulating layer 160 may contain La, nitrogen, Zr, or the like as an impurity.

An example of a layered structure of the insulating layer 160 is described. The insulating layer 160 includes, for example, oxygen, nitrogen, silicon, or hafnium. Specifically, the insulating layer 160 preferably includes hafnium oxide and silicon oxide or silicon oxynitride.

Hafnium oxide and aluminum oxide have higher dielectric constants than silicon oxide and silicon oxynitride. Therefore, the insulating layer 160 using hafnium oxide or aluminum oxide can have larger thickness than the insulating layer 160 using silicon oxide, so that leakage current due to tunnel current can be reduced. That is, a transistor with low off-state current can be provided. Moreover, hafnium oxide with a crystalline structure has a higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with low off-state current. Examples of the crystal structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to the above examples.

For the insulating layers 120 and 160 in contact with the oxide semiconductor layer 130, a film that releases less nitrogen oxide is preferably used. For the insulating layers 120 and 160, for example, a silicon oxynitride film or an aluminum oxynitride film that releases less nitrogen oxide can be used.

A silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in TDS; the amount of released ammonia is typically greater than or equal to 1×10¹⁸ molecules/cm³ and less than or equal to 5×10¹⁹ molecules/cm³. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of the film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C.

By using the above oxide insulating layer for the insulating layers 120 and 160, a shift in the threshold voltage of the transistor can be reduced, which leads to reduced fluctuations in the electrical characteristics of the transistor.

For the conductive layer 170 functioning as a gate electrode layer, for example, a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Mn, Nd, Sc, Ta, W, or the like can be used. Alternatively, an alloy or a conductive nitride of any of these materials may be used. Alternatively, a stack of a plurality of materials selected from these materials, alloys of these materials, and conductive nitrides of these materials may be used. Typically, tungsten, a stack of tungsten and titanium nitride, a stack of tungsten and tantalum nitride, or the like can be used. Alternatively, Cu or an alloy such as Cu—Mn, which has low resistance, or a stack of any of the above materials and Cu or an alloy such as Cu—Mn may be used. In this embodiment, tantalum nitride is used for the conductive layer 171 and tungsten is used for the conductive layer 172 to form the conductive layer 170.

As the insulating layer 175, a silicon nitride film, an aluminum nitride film, or the like containing hydrogen can be used. In the transistors 103, 104, 106, 109, 110, and 112 described in Embodiment 4, when an insulating film containing hydrogen is used as the insulating layer 175, part of the oxide semiconductor layer can have n-type conductivity. In addition, a nitride insulating film functions as a blocking film against moisture and the like and can improve the reliability of the transistor.

An aluminum oxide film can also be used as the insulating layer 175. It is particularly preferable to use an aluminum oxide film as the insulating layer 175 in the transistors 101, 102, 105, 107, 108, and 111 described in Embodiment 4. The aluminum oxide film has a high blocking effect of preventing penetration of both oxygen and impurities such as hydrogen and moisture. Accordingly, during and after the manufacturing process of the transistor, the aluminum oxide film can suitably function as a protective film that has effects of preventing entry of impurities such as hydrogen and moisture into the oxide semiconductor layer 130, preventing release of oxygen from the oxide semiconductor layer, and preventing unnecessary release of oxygen from the insulating layer 120. Furthermore, oxygen contained in the aluminum oxide film can be diffused into the oxide semiconductor layer.

Furthermore, the insulating layer 180 is preferably formed over the insulating layer 175. The insulating layer 180 can be formed using an insulating film containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. The insulating layer 180 may be a stack of any of the above materials.

Here, like the insulating layer 120, the insulating layer 180 preferably contains oxygen more than that in the stoichiometric composition. Oxygen released from the insulating layer 180 can be diffused into the channel formation region in the oxide semiconductor layer 130 through the insulating layer 160, so that oxygen vacancies formed in the channel formation region can be filled with oxygen. In this manner, stable electrical characteristics of the transistor can be achieved.

High integration of a semiconductor device requires miniaturization of a transistor. However, it is known that miniaturization of a transistor causes deterioration of electrical characteristics of the transistor. In particular, a decrease in channel width causes a reduction in on-state current.

In the transistors 107 to 112 in one embodiment of the present invention, the oxide semiconductor layer 130 c is formed to cover the oxide semiconductor layer 130 b where a channel is formed; thus, a channel formation layer is not in contact with the gate insulating film. Accordingly, scattering of carriers at the interface between the channel formation layer and the gate insulating film can be reduced and the on-state current of the transistor can be increased.

In the transistor in one embodiment of the present invention, as described above, the gate electrode layer (the conductive layer 170) is formed to electrically surround the oxide semiconductor layer 130 in the channel width direction; accordingly, a gate electric field is applied to the oxide semiconductor layer 130 in a direction perpendicular to its side surface in addition to a direction perpendicular to its top surface. In other words, a gate electric field is applied to the entire channel formation layer and effective channel width is increased, leading to a further increase in the on-state current.

Furthermore, in the transistor in one embodiment of the present invention in which the oxide semiconductor layer 130 has a two-layer structure or a three-layer structure, since the oxide semiconductor layer 130 b where a channel is formed is provided over the oxide semiconductor layer 130 a, an effect of making an interface state less likely to be formed is obtained. In the transistor in one embodiment of the present invention in which the oxide semiconductor layer 130 has a three-layer structure, since the oxide semiconductor layer 130 b is positioned at the middle of the three-layer structure, an effect of eliminating the influence of an impurity that enters from upper and lower layers on the oxide semiconductor layer 130 b is obtained as well. Therefore, the transistor can achieve not only the increase in the on-state current of the transistor but also stabilization of the threshold voltage and a reduction in the S value (subthreshold value). Thus, current when gate voltage VG is 0 V can be reduced and power consumption can be reduced. In addition, since the threshold voltage of the transistor becomes stable, long-term reliability of the semiconductor device can be improved. Furthermore, the transistor in one embodiment of the present invention is suitable for a highly integrated semiconductor device because deterioration of electrical characteristics due to miniaturization is reduced.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments and Example as appropriate.

Embodiment 6

In this embodiment, methods for manufacturing the transistors 101, 107, and 111 described in Embodiment 4 are described.

First, a method for manufacturing a silicon transistor included in the substrate 115 is described. Here, an example of a method for manufacturing a p-ch transistor is described. An n⁻-type single crystal silicon substrate is used as a silicon substrate, and an element formation region isolated with an insulating layer (also referred to as a field oxide film) is formed in the surface. The element formation region can be formed by local oxidation of silicon (LOCOS), shallow trench isolation (STI), or the like.

Here, the substrate is not limited to the single crystal silicon substrate. A silicon on insulator (SOI) substrate or the like can also be used.

Next, a gate insulating film is formed to cover the element formation region. For example, a silicon oxide film is formed by oxidation of a surface of the element formation region by heat treatment. Furthermore, after the silicon oxide film is formed, a surface of the silicon oxide film may be nitrided by nitriding treatment.

Next, a conductive film is formed to cover the gate insulating film. The conductive film can be formed using an element selected from Ta, W, Ti, Mo, Al, Cu, Cr, Nb, and the like, or an alloy material or a compound material containing such an element as a main component. Alternatively, a metal nitride film obtained by nitriding of any of these elements can be used. Alternatively, a semiconductor material typified by polycrystalline silicon doped with an impurity element such as phosphorus can be used.

Then, the conductive film is selectively etched, so that a gate electrode layer is formed over the gate insulating film.

Next, an insulating film such as a silicon oxide film or a silicon nitride film is formed to cover the gate electrode layer and etch back is performed, so that sidewalls are formed on side surfaces of the gate electrode layer.

Next, a resist mask is selectively formed to cover regions except the element formation region, and an impurity element is added using the resist mask and the gate electrode layer as masks, so that pt-type impurity regions are formed. Here, in order to form a p-ch transistor, an impurity element imparting p-type conductivity such as B or Ga can be used as the impurity element.

Through the above steps, a p-ch transistor including an active region in the silicon substrate is completed. Note that a passivation film such as a silicon nitride film or an aluminum oxide film is preferably formed over the transistor.

Next, an interlayer insulating film is formed over the silicon substrate where the transistor is formed, and contact plugs and wirings are formed.

A method for manufacturing the transistor 101 is described with reference to FIGS. 33A to 33C and FIGS. 34A to 34C. A cross section of the transistor in the channel length direction is shown on the left side, and a cross section of the transistor in the channel width direction is shown on the right side. The cross-sectional views in the channel width direction are enlarged views; therefore, components on the left side and those on the right side differ in apparent thickness.

The case where the oxide semiconductor layer 130 has a three-layer structure of the oxide semiconductor layers 130 a to 130 c is described as an example. In the case where the oxide semiconductor layer 130 has a two-layer structure, the oxide semiconductor layers 130 a and 130 b are used. In the case where the oxide semiconductor layer 130 has a single-layer structure, the oxide semiconductor layer 130 b is used.

First, the insulating layer 120 is formed over the substrate 115. Embodiment 5 can be referred to for the kind of the substrate 115 and the material of the insulating layer 120. The insulating layer 120 can be formed by sputtering, CVD, molecular beam epitaxy (MBE), or the like.

Oxygen may be added to the insulating layer 120 by ion implantation, ion doping, plasma immersion ion implantation, plasma treatment, or the like. Adding oxygen enables the insulating layer 120 to supply oxygen much easily to the oxide semiconductor layer 130.

In the case where a surface of the substrate 115 is made of an insulator and there is no influence of impurity diffusion on the oxide semiconductor layer 130 to be formed later, the insulating layer 120 is not necessarily provided.

Next, an oxide semiconductor film 130A to be the oxide semiconductor layer 130 a, an oxide semiconductor film 130B to be the oxide semiconductor layer 130 b, and an oxide semiconductor film 130C to be the oxide semiconductor layer 130 c are formed over the insulating layer 120 by sputtering, CVD, MBE, or the like (see FIG. 33A).

In the case where the oxide semiconductor layer 130 has a layered structure, oxide semiconductor films are preferably formed successively without exposure to the air with the use of a multi-chamber deposition apparatus (e.g., a sputtering apparatus) including a load lock chamber. It is preferable that each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (approximately 5×10⁻⁷ to 1×10⁻⁴ Pa) by an adsorption vacuum evacuation pump such as a cryopump and that the chamber be able to heat the substrate to higher than or equal to 100° C., preferably higher than or equal to 500° C., so that water and the like serving as impurities of an oxide semiconductor are removed as much as possible. Alternatively, the combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber. Alternatively, the combination of a turbo molecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation of the chamber but also high purity of a sputtering gas is preferred to obtain a highly purified intrinsic oxide semiconductor. As an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, more preferably −100° C. or lower is used, so that entry of moisture or the like into the oxide semiconductor film can be prevented as much as possible.

For the oxide semiconductor films 130A to 130C, any of the materials described in Embodiment 5 can be used. In the case where sputtering is used for deposition, any of the materials described in Embodiment 5 can be used as a target.

Note that as described in detail in Embodiment 5, a material that has a higher electron affinity than the oxide semiconductor films 130A and 130C is used for the oxide semiconductor film 130B.

The oxide semiconductor films are preferably formed by sputtering. As sputtering, RF sputtering, DC sputtering, AC sputtering, or the like can be used.

After the oxide semiconductor film 130C is formed, first heat treatment may be performed. The first heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C., in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced pressure state. Alternatively, the first heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, in order to compensate released oxygen. The first heat treatment can increase the crystallinity of the oxide semiconductor films 130A to 130C and remove impurities such as water and hydrogen from the insulating layer 120 and the oxide semiconductor films 130A to 130C. Note that the first heat treatment may be performed after etching for forming the oxide semiconductor layers 130 a to 130 c described later.

Next, a conductive layer is formed over the oxide semiconductor film 130C. The conductive layer can be, for example, formed by the following method.

First, a first conductive film is formed over the oxide semiconductor film 130C. As the first conductive film, a single layer or a stacked layer can be formed using a material selected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and an alloy of any of these metal materials.

Next, a resist film is formed over the first conductive film and the resist film is exposed to light by electron beam exposure, liquid immersion exposure, or EUV exposure and developed, so that a first resist mask is formed. An organic coating film is preferably formed as an adherence agent between the first conductive film and the resist film. Alternatively, the first resist mask may be formed by nanoimprint lithography.

Then, the first conductive film is selectively etched using the first resist mask and the first resist mask is subjected to ashing; thus, the conductive layer is formed.

Next, the oxide semiconductor films 130A to 130C are selectively etched using the conductive layer as a hard mask and the conductive layer is removed; thus, the oxide semiconductor layer 130 including a stack of the oxide semiconductor layers 130 a to 130 c is formed (see FIG. 33B). It is also possible to form the oxide semiconductor layer 130 using the first resist mask, without forming the conductive layer. Here, oxygen ions may be implanted into the oxide semiconductor layer 130.

Next, a second conductive film is formed to cover the oxide semiconductor layer 130. The second conductive film can be formed using a material that can be used for the conductive layers 140 and 150 described in Embodiment 5. Sputtering, CVD, MBE, or the like can be used for the formation of the second conductive film.

Then, a second resist mask is formed over portions to be a source region and a drain region. Then, part of the second conductive film is etched, so that the conductive layers 140 and 150 are formed (see FIG. 33C).

Next, an insulating film 160A is formed over the oxide semiconductor layer 130 and the conductive layers 140 and 150. The insulating film 160A can be formed using a material that can be used for the insulating layer 160 described in Embodiment 5. Sputtering, CVD, MBE, or the like can be used for the formation of the insulating film 160A.

After that, second heat treatment may be performed. The second heat treatment can be performed in a condition similar to that of the first heat treatment. The second heat treatment can make oxygen diffuse from the insulating layer 120 into the entire oxide semiconductor layer 130. Note that it is possible to obtain this effect by third heat treatment, without performing the second heat treatment.

Then, a third conductive film 171A and a fourth conductive film 172A to be the conductive layer 170 are formed over the insulating film 160A. The third conductive film 171A and the fourth conductive film 172A can be formed using materials that can be used for the conductive layers 171 and 172 described in Embodiment 5. Sputtering, CVD, MBE, or the like can be used for the formation of the third conductive film 171A and the fourth conductive film 172A.

Next, a third resist mask 156 is formed over the fourth conductive film 172A (see FIG. 34A). The third conductive film 171A, the fourth conductive film 172A, and the insulating film 160A are selectively etched using the third resist mask 156, so that the conductive layer 170 including the conductive layers 171 and 172 and the insulating layer 160 are formed (see FIG. 34B). Note that if the insulating film 160A is not etched, the transistor 102 can be manufactured.

After that, the insulating layer 175 is formed over the oxide semiconductor layer 130, the conductive layers 140 and 150, the insulating layer 160, and the conductive layer 170. Embodiment 5 can be referred to for the material of the insulating layer 175. In the transistor 101, an aluminum oxide film is preferably used. The insulating layer 175 can be formed by sputtering, CVD, MBE, or the like.

Next, the insulating layer 180 is formed over the insulating layer 175 (see FIG. 34C). Embodiment 5 can be referred to for the material of the insulating layer 180. The insulating layer 180 can be formed by sputtering, CVD, MBE, or the like.

Oxygen may be added to the insulating layer 175 and/or the insulating layer 180 by ion implantation, ion doping, plasma immersion ion implantation, plasma treatment, or the like. Adding oxygen enables the insulating layer 175 and/or the insulating layer 180 to supply oxygen much easily to the oxide semiconductor layer 130.

Next, the third heat treatment may be performed. The third heat treatment can be performed in a condition similar to that of the first heat treatment. By the third heat treatment, excess oxygen is easily released from the insulating layers 120, 175, and 180, so that oxygen vacancies in the oxide semiconductor layer 130 can be reduced.

Next, a method for manufacturing the transistor 107 is described. Note that detailed description of steps similar to those for manufacturing the transistor 102 described above is omitted.

The insulating layer 120 is formed over the substrate 115, and the oxide semiconductor film 130A to be the oxide semiconductor layer 130 a and the oxide semiconductor film 130B to be the oxide semiconductor layer 130 b are formed over the insulating layer 120 by sputtering, CVD, MBE, or the like (see FIG. 35A).

After that, a first conductive film is formed over the oxide semiconductor film 130B, and a conductive layer is formed using a first resist mask by a method similar to the above method. Then, the oxide semiconductor films 130A and 130B are selectively etched using the conductive layer as a hard mask and the conductive layer is removed; thus, a stack of the oxide semiconductor layers 130 a and 130 b is formed (see FIG. 35B). It is also possible to form the stack using the first resist mask, without forming the hard mask. Here, oxygen ions may be implanted into the oxide semiconductor layers 130 a and 130 b.

Next, a second conductive film is formed to cover the stack. Then, a second resist mask is formed over portions to be a source region and a drain region, and part of the second conductive film is etched using the second resist mask, so that the conductive layers 140 and 150 are formed (see FIG. 35C).

After that, the oxide semiconductor film 130C to be the oxide semiconductor layer 130 c is formed over the stack of the oxide semiconductor layers 130 a and 130 b and the conductive layers 140 and 150. Furthermore, the insulating film 160A, the third conductive film 171A, and the fourth conductive film 172A are formed over the oxide semiconductor film 130C.

Then, the third resist mask 156 is formed over the fourth conductive film 172A (see FIG. 36A). The third conductive film 171A, the fourth conductive film 172A, the insulating film 160A, and the oxide semiconductor film 130C are selectively etched using the resist mask, so that the conductive layer 170 including the conductive layers 171 and 172, the insulating layer 160, and the oxide semiconductor layer 130 c are formed (see FIG. 36B). Note that when the insulating film 160A and the oxide semiconductor film 130C are etched using a fourth resist mask, the transistor 108 can be manufactured.

Next, the insulating layers 175 and 180 are formed over the insulating layer 120, the oxide semiconductor layer 130 (the oxide semiconductor layers 130 a to 130 c), the conductive layers 140 and 150, the insulating layer 160, and the conductive layer 170 (see FIG. 36C).

Through the above steps, the transistor 107 can be manufactured.

Next, a method for manufacturing the transistor 111 is described. Note that detailed description of steps similar to those for manufacturing the transistor 102 described above is omitted.

The insulating layer 120 is formed over the substrate 115, and the oxide semiconductor film 130A to be the oxide semiconductor layer 130 a and the oxide semiconductor film 130B to be the oxide semiconductor layer 130 b are formed over the insulating layer 120 by sputtering, CVD, MBE, or the like. Then, a first conductive film is formed over the oxide semiconductor film 130B, and a conductive layer 141 a is formed using a first resist mask (see FIG. 37A).

Then, the oxide semiconductor films 130A and 130B are selectively etched using the conductive layer 141 a as a hard mask, so that a stack of the oxide semiconductor layers 130 a and 130 b and the conductive layer 141 a is formed (see FIG. 37B). Here, oxygen ions may be implanted into the oxide semiconductor layers 130 a and 130 b.

Then, a second resist mask is formed over portions to be a source region and a drain region, and part of the conductive layer 141 a is etched using the second resist mask, so that the conductive layers 141 and 151 are formed (see FIG. 37C).

After that, the oxide semiconductor film 130C to be the oxide semiconductor layer 130 c is formed over the stack of the oxide semiconductor layers 130 a and 130 b and the conductive layers 141 and 151. Furthermore, the insulating film 160A, the third conductive film 171A, and the fourth conductive film 172A are formed over the oxide semiconductor film 130C.

Then, the third resist mask 156 is formed over the fourth conductive film 172A (see FIG. 38A). The third conductive film 171A, the fourth conductive film 172A, the insulating film 160A, and the oxide semiconductor film 130C are selectively etched using the third resist mask 156, so that the conductive layer 170 including the conductive layers 171 and 172, the insulating layer 160, and the oxide semiconductor layer 130 c are formed (see FIG. 38B).

Next, the insulating layers 175 and 180 are formed over the insulating layer 120, the oxide semiconductor layer 130 (the oxide semiconductor layers 130 a to 130 c), the conductive layers 140 and 150, the insulating layer 160, and the conductive layer 170.

Next, openings reaching the conductive layers 141 and 151 are provided in the insulating layers 175 and 180, and a fifth conductive film is formed to cover the openings. Then, a fourth resist mask is provided over the fifth conductive film and the fifth conductive film is selectively etched using the resist mask, so that the conductive layers 142 and 152 are formed (see FIG. 38C).

Through the above steps, the transistor 111 can be manufactured.

Although the variety of films such as the metal films, the semiconductor films, and the inorganic insulating films that are described in this embodiment typically can be formed by sputtering or plasma-enhanced CVD, such films may be formed by another method such as thermal CVD. Examples of thermal CVD include metal organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD).

Since plasma is not used for deposition, thermal CVD has an advantage that no defect due to plasma damage is generated.

Deposition by thermal CVD may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at the same time, the pressure in the chamber is set to an atmospheric pressure or a reduced pressure, and reaction is caused in the vicinity of the substrate or over the substrate.

Deposition by ALD is performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are introduced into the chamber and reacted, and then the sequence of gas introduction is repeated. An inert gas (e.g., argon or nitrogen) may be introduced as a carrier gas with the source gases. For example, two or more kinds of source gases may be sequentially supplied to the chamber. In that case, after reaction of a first source gas, an inert gas is introduced, and then a second source gas is introduced so that the source gases are not mixed. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate and reacted to form a first layer, and then, the second source gas introduced is absorbed and reacted. As a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of gas introduction is controlled and repeated more than once until desired thickness is obtained, so that a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of gas introduction; therefore, ALD makes it possible to accurately adjust thickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, and the inorganic insulating film that have been disclosed in the embodiments can be formed by thermal CVD such as MOCVD or ALD. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium (In(CH₃)₃), trimethylgallium (Ga(CH₃)₃), and dimethylzinc (Zn(CH₃)₂) can be used. The chemical formula of trimethylgallium is Ga(CH₃)₃. Without limitation to the above combination, triethylgallium (Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc (Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by a deposition apparatus using ALD, two kinds of gases, i.e., ozone (O₃) as an oxidizer and a source material gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor (hafnium alkoxide and a hafnium amide such as hafnium tetrakis(dimethylamide)hafnium (TDMAH, Hf[N(CH₃)₂]₄) and tetrakis(ethylmethylamide)hafnium) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by a deposition apparatus using ALD, two kinds of gases, i.e., H₂O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor (e.g., trimethylaluminum (TMA, Al(CH₃)₃)) are used. Examples of another material include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by a deposition apparatus using ALD, hexachlorodisilane is adsorbed on a surface where a film is to be formed, and radicals of an oxidizing gas (e.g., O₂ or dinitrogen monoxide) are supplied to react with an adsorbate.

For example, in the case where a tungsten film is formed by a deposition apparatus using ALD, a WF₆ gas and a B₂H₆ gas are sequentially introduced to form an initial tungsten film, and then a WF₆ gas and an H₂ gas are sequentially introduced to form a tungsten film. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed by a deposition apparatus using ALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are sequentially introduced to form a Ga—O layer, and then a Zn(CH₃)₂ gas and an O₃ gas are sequentially introduced to form a Zn—O layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by using these gases. Although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

Embodiment 7

The structure of an oxide semiconductor film that can be used for one embodiment of the present invention is described below.

In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” indicates that an angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system.

An oxide semiconductor film is roughly classified into a non-single-crystal oxide semiconductor film and a single-crystal oxide semiconductor film. The non-single-crystal oxide semiconductor film means any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, a polycrystalline oxide semiconductor film, a microcrystalline oxide semiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a plurality of c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of the CAAC-OS film is observed. Consequently, a plurality of crystal parts are observed clearly. However, in the high-resolution TEM image, a boundary between crystal parts, i.e., a grain boundary is not observed clearly. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the high-resolution cross-sectional TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology that reflects a surface over which the CAAC-OS film is formed (also referred to as a formation surface) or a top surface of the CAAC-OS film, and is provided parallel to the formation surface or the top surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (planar TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

The CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2 θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 2 θ may also be observed at around 36°, in addition to the peak of 2 θ at around 31°. The peak of 2 θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2 θ appear at around 31° and a peak of 2 θ not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor film, such as silicon, disturbs the atomic order of the oxide semiconductor film by depriving the oxide semiconductor film of oxygen and causes a decrease in crystallinity. Furthermore, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic order of the oxide semiconductor film and causes a decrease in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor film might serve as a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic.” A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have low carrier density. Thus, a transistor including the oxide semiconductor film rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has few variations in electrical characteristics and high reliability. Charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released and may behave like fixed charge. Thus, the transistor that includes the oxide semiconductor film having high impurity concentration and high density of defect states has unstable electrical characteristics in some cases.

In a transistor including the CAAC-OS film, changes in electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light are small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystal part is observed in a high-resolution TEM image and a region where a crystal part is not clearly observed in a high-resolution TEM image. In most cases, a crystal part in the microcrystalline oxide semiconductor film is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor film including nanocrystal is referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In a high-resolution TEM image, a crystal grain boundary cannot be found clearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has periodic atomic order. There is no regularity of crystal orientation between different crystal parts in the nc-OS film. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor film depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak that shows a crystal plane does not appear. Furthermore, a halo pattern is shown in a selected-area electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter larger than the diameter of a crystal part (e.g., larger than or equal to 50 nm). Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter close to or smaller than the diameter of a crystal part. Furthermore, in a nanobeam electron diffraction pattern of the nc-OS film, circumferentially distributed spots are observed in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots are shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularity than an amorphous oxide semiconductor film. Thus, the nc-OS film has a lower density of defect states than the amorphous oxide semiconductor film. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film; thus, the nc-OS film has a higher density of defect states than the CAAC-OS film.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangement and no crystal part. For example, the amorphous oxide semiconductor film does not have a specific state as in quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor film, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak which shows a crystal plane does not appear. A halo pattern is shown in an electron diffraction pattern of the amorphous oxide semiconductor film. Furthermore, a halo pattern is shown but a spot is not shown in a nanobeam electron diffraction pattern of the amorphous oxide semiconductor film.

Note that an oxide semiconductor film may have a structure having physical properties between the nc-OS film and the amorphous oxide semiconductor film. The oxide semiconductor film having such a structure is specifically referred to as an amorphous-like oxide semiconductor (a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may be seen. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. In the a-like OS film, crystallization by a slight amount of electron beam used for TEM observation occurs and growth of the crystal part is found sometimes. In contrast, crystallization by a slight amount of electron beam used for TEM observation is less observed in the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS film can be measured using high-resolution TEM images. For example, an InGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers are included between In—O layers. A unit cell of the InGaZnO₄ crystal has a structure in which nine layers of three In—O layers and six Ga—Zn—O layers are layered in the c-axis direction. Accordingly, the spacing between these adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as a d value). The value is calculated to be 0.29 nm from crystal structure analysis. Thus, each of the lattice fringes in which the spacing therebetween is from 0.28 nm to 0.30 nm corresponds to the a-b plane of the InGaZnO₄ crystal, focusing on the lattice fringes in the high-resolution TEM image.

Note that an oxide semiconductor film may be a stacked film including two or more films of an amorphous oxide semiconductor film, an a-like OS film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

Embodiment 8

An imaging device in one embodiment of the present invention and a semiconductor device including the imaging device can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices that reproduce the content of recording media such as digital versatile discs (DVD) and have displays for displaying the reproduced images). Furthermore, as electronic devices that can include the imaging device in one embodiment of the present invention and the semiconductor device including the imaging device, cellular phones, game machines (including portable game machines), portable information terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), vending machines, and the like can be given. FIGS. 39A to 39F illustrate specific examples of these electronic devices.

FIG. 39A illustrates a portable game machine, which includes housings 901 and 902, display portions 903 and 904, a microphone 905, speakers 906, an operation key 907, a stylus 908, a camera 909, and the like. Although the portable game machine in FIG. 39A has the two display portions 903 and 904, the number of display portions included in the portable game machine is not limited to this. The imaging device in one embodiment of the present invention can be used for the camera 909.

FIG. 39B illustrates a portable data terminal, which includes a first housing 911, a display portion 912, a camera 919, and the like. The touch panel function of the display portion 912 enables input and output of information. The imaging device in one embodiment of the present invention can be used for the camera 919.

FIG. 39C illustrates a digital camera, which includes a housing 921, a shutter button 922, a microphone 923, a light-emitting portion 927, a lens 925, and the like. The imaging device in one embodiment of the present invention can be provided in a focus position of the lens 925.

FIG. 39D illustrates a wrist-watch-type information terminal, which includes a housing 931, a display portion 932, a wristband 933, a camera 939, and the like. The display portion 932 may be a touch panel. The imaging device in one embodiment of the present invention can be used for the camera 939.

FIG. 39E illustrates a video camera, which includes a first housing 941, a second housing 942, a display portion 943, operation keys 944, a lens 945, a joint 946, and the like. The operation keys 944 and the lens 945 are provided for the first housing 941, and the display portion 943 is provided for the second housing 942. The first housing 941 and the second housing 942 are connected to each other with the joint 946, and an angle between the first housing 941 and the second housing 942 can be changed with the joint 946. An image displayed on the display portion 943 may be switched in accordance with the angle between the first housing 941 and the second housing 942 at the joint 946. The imaging device in one embodiment of the present invention can be provided in a focus position of the lens 945.

FIG. 39F illustrates a cellular phone, which includes a display portion 952, a microphone 957, a speaker 954, a camera 959, an input/output terminal 956, an operation button 955, and the like in a housing 951. The imaging device in one embodiment of the present invention can be used for the camera 959.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments and Example as appropriate.

Example

In this example, imaging devices in one embodiment of the present invention described in the above embodiment are fabricated and operation results of the imaging devices are described.

FIG. 43 illustrates an overview of the structure of the imaging device. A motion detector and an A/D converter operate exclusively; therefore, the imaging device can perform motion capturing and normal capturing. A block diagram of the motion detector is illustrated in FIG. 44, and its timing chart illustrating motion capturing operation is shown in FIG. 45. The motion detector compares pixel output with reference voltage using a comparator in each column. The data of a reference frame is stored in a nonvolatile analog memory in each pixel, and differenctial data between the reference frame and a current frame is calculated in the pixel. When there is no difference between the reference frame and the current frame, the pixel output corresponds to a reference output level. On the other hand, when there is difference between the reference frame and the current frame, the motion detector detects pixel output that exceeds the reference voltage range from V_(REF+) to V_(REF−), and then the motion detector outputs a 1-bit motion trigger.

FIG. 46 illustrates the structure of a pixel circuit with a threshold voltage compensation function in the imaging device. Embodiment 1 can be referred to for operation of the pixel.

Timing charts illustrating motion capturing operation are described in FIG. 47. First, in the reference frame, an object is exposed to light under the condition where the transistor M3 is on and the potential of the node FD2 is fixed. That is, the potential of the node FD2 is initialized. Furthermore, data of the reference frame corresponds to the change in the potential of the node FD1. On the other hand, in the target frame, the object is exposed to light under the condition where the transistor M3 is off and the node FD2 is floating. Accordingly, when data of the target frame is the same as the data of the reference frame, the potential of the node FD2 is returned to the initial potential after capturing, and the pixel output level is the same as the reference output level. In contrast, when the data of the target frame is different from the data of the reference frame, the potential of the node FD2 is changed depending on the differential data, and the pixel output level is different from the reference output level.

The imaging device in this example is fabricated using CAAC-OS transistors and Si transistors. The specifications of the imaging device are listed in Table 1, and a chip micrograph of the imaging device is shown in FIG. 48.

TABLE 1 w/o V_(th) compensation w/V_(th) compensation circuit circuit Process 0.5 μm n-ch CAAC-OS technology 0.18 μm p-ch Si Die size 6.5 mm × 6.5 mm Number of pixels 240 × 160 Pixel size 20 μm × 20 μm Pixel configuration 5 transistors, 7 transistors, 2 capacitors 4 capacitors Fill factor 29.75% 28.25% FPN  1.73%  1.45% Contrast sensitivity 200 mV 97.5 mV at 120 of the at 120 of the output value output value Power supply I/O: 1.8 V pixel, driver, ADC, analog: 3.3 V ADC 8-bit single slope

FIG. 49 is an output value (data after 8-bit A/D conversion) graph of all the pixels when the potential of the node FD1 is changed through the transistor M3. In FIG. 49, output value graphs of imaging devices with and without threshold voltage compensation circuits are compared. The graphs indicate that the uniformity of pixel output of the imaging device with the threshold voltage compensation circuit is increased at each output value.

The calculated standard deviation between the pixels at each output value is shown in FIG. 50. The imaging device with the threshold voltage compensation circuit reduces variation between the pixels by 1.39 at 120 of the output value, for example. Note that it is confirmed that output gain variation of the imaging device with the threshold voltage compensation is increased by 37.8% compared with the imaging device without the threshold voltage compensation, and that the output offset variation is reduced by 22.2%. The total variation of the imaging device with the threshold voltage compensation circuit is reduced, though the gain variation might be increased because of the increase in the number of pixel transistors and capacitance variation.

According to fixed pattern noise (FPN) calculated from variation between the pixels in FIG. 50, the threshold voltage compensation circuit reduces FPN by 16.2%.

FIG. 51 shows normal captured images of the imaging devices with and without the threshold voltage compensation circuits. Threshold voltage compensation operation of the imaging device with the threshold voltage compensation circuit produces similar results to correlated double sampling (CDS) and is effective in removing FPN.

The capturing accuracies of the motion detector with and without the threshold voltage compensation circuits are compared. The closest values of the reference voltages V_(REF+) and V_(REF−) of the comparator outputting a motion trigger of the motion detector are measured by setting the pixel at a certain reference output level. As ΔV_(REF)=V_(REF+)−V_(REF−) becomes smaller, the motion detector can detect even a small difference and can reduce detection errors. FIG. 52 shows results. At 120 of the output value, for example, the threshold voltage compensation circuit reduces ΔV_(REF) from 200 mV to 97.5 mV. In other words, the detectable range of reference voltage is reduced to 1/2.05. This offers a 2.05-fold improvement in capturing sensitivity.

FIG. 53 illustrates motion capturing results when an object of the reference frame is a transparent film and an object of the target frame is a character-printed film. Here, the experiments are conducted using films with printed characters of different depths of colors as a plurality of objects. The imaging device can capture a difference when there is a tiny difference between the depths of the colors, as illustrated in FIG. 53. Note that the amount of variation between the pixels after one hour of threshold voltage compensation operation is confirmed to be approximately 8%. That is, if threshold voltage compensation is repeated very infrequently, for example, once every few hours, the threshold voltage compensation effect can be maintained. This is due to retention of the potential of the capacitor C4 by the extremely low off-state current of the CAAC-OS transistor.

As described in this example, when a pixel circuit has a threshold voltage compensation function, FPN of an imaging device can be reduced by 16.2%. In addition, a 2.05-fold improvement in motion-capturing sensitivity can be offered. Consequently, the imaging device can capture motion even when the color of an object is changed slightly.

Note that this example can be combined with any of the embodiments described in this specification as appropriate.

REFERENCE NUMERALS

11: circuit, 12: circuit, 21: wiring, 22: wiring, 23: wiring, 24: wiring, 25: wiring, 26: wiring, 27: wiring, 28: wiring, 29: wiring, 30: wiring, 31: wiring, 32: wiring, 40: silicon substrate, 51: transistor, 52: transistor, 53: transistor, 54: transistor, 55: transistor, 56: transistor, 57: transistor, 58: transistor, 59: transistor, 60: photodiode, 70: transistor, 71: transistor, 80: insulating layer, 90: circuit portion, 92: circuit portion, 101: transistor, 102: transistor, 103: transistor, 104: transistor, 105: transistor, 106: transistor, 107: transistor, 108: transistor, 109: transistor, 110: transistor, 111: transistor, 112: transistor, 115: substrate, 120: insulating layer, 130: oxide semiconductor layer, 130 a: oxide semiconductor layer, 130A: oxide semiconductor film, 130 b: oxide semiconductor layer, 130B: oxide semiconductor film, 130 c: oxide semiconductor layer, 130C: oxide semiconductor film, 140: conductive layer, 141: conductive layer, 141 a: conductive layer, 142: conductive layer, 150: conductive layer, 151: conductive layer, 152: conductive layer, 156: resist mask, 160: insulating layer, 160A: insulating film, 170: conductive layer, 171: conductive layer, 171A: conductive film, 172: conductive layer, 172A: conductive film, 173: conductive layer, 175: insulating layer, 180: insulating layer, 231: region, 232: region, 233: region, 331: region, 332: region, 333: region, 334: region, 335: region, 400: pixel portion, 410: row driver, 420: A/D converter, 430: column driver, 501: signal, 502: signal, 503: signal, 504: signal, 505: signal, 506: signal, 507: signal, 508: signal, 509: signal, 510: period, 511: period, 520: period, 531: period, 610: period, 611: period, 612: period, 613: period, 621: period, 622: period, 623: period, 631: period, 901: housing, 902: housing, 903: display portion, 904: display portion, 905: microphone, 906: speaker, 907: operation key, 908: stylus, 909: camera, 911: housing, 912: display portion, 919: camera, 921: housing, 922: shutter button, 923: microphone, 925: lens, 927: light-emitting portion, 931: housing, 932: display portion, 933: wristband, 939: camera, 941: housing, 942: housing, 943: display portion, 944: operation key, 945: lens, 946: joint, 951: housing, 952: display portion, 954: speaker, 955: button, 956: input/output terminal, 957: microphone, and 959: camera.

This application is based on Japanese Patent Application serial No. 2014-176012 filed with Japan Patent Office on Aug. 29, 2014 and Japanese Patent Application serial No. 2015-009962 filed with Japan Patent Office on Jan. 22, 2015, the entire contents of which are hereby incorporated by reference. 

1. An imaging device comprising: a photoelectric conversion element; a first transistor; a second transistor; a third transistor; a fourth transistor; a fifth transistor; a sixth transistor; a seventh transistor; an eighth transistor; a first capacitor; and a second capacitor, wherein one terminal of the photoelectric conversion element is electrically connected to one of a source and a drain of the first transistor, wherein the other of the source and the drain of the first transistor is electrically connected to one of a source and a drain of the seventh transistor, wherein the other of the source and the drain of the first transistor is electrically connected to one terminal of the first capacitor, wherein one of a source and a drain of the second transistor is electrically connected to the other terminal of the first capacitor, wherein the other terminal of the first capacitor is electrically connected to one terminal of the second capacitor, wherein one of a source and a drain of the third transistor is electrically connected to the other terminal of the second capacitor, wherein the other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor, wherein one of a source and a drain of the fifth transistor is electrically connected to the one of the source and the drain of the fourth transistor, wherein a gate of the fifth transistor is electrically connected to the one of the source and the drain of the third transistor, wherein one of a source and a drain of the sixth transistor is electrically connected to the other of the source and the drain of the fifth transistor, and wherein the other of the source and the drain of the sixth transistor is electrically connected to one of a source and a drain of the eighth transistor.
 2. The imaging device according to claim 1, further comprising a ninth transistor, wherein one of a source and a drain of the ninth transistor is electrically connected to the other of the source and the drain of the eighth transistor, wherein a gate of the ninth transistor is electrically connected to a gate of the eighth transistor, and wherein the gate of the ninth transistor is electrically connected to the other of the source and the drain of the ninth transistor.
 3. The imaging device according to claim 1, wherein the other of the source and the drain of the second transistor is electrically connected to the other terminal of the photoelectric conversion element.
 4. The imaging device according to claim 1, further including a fourth capacitor, wherein one terminal of the fourth capacitor is electrically connected to the one of the source and the drain of the second transistor.
 5. The imaging device according to claim 1, wherein each of the first to eighth transistors includes an oxide semiconductor in an active layer, and wherein the oxide semiconductor includes In, Zn, and M, where M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf.
 6. The imaging device according to claim 1, further comprising a third capacitor, wherein one terminal of the third capacitor is electrically connected to the other terminal of the second capacitor, and wherein the other terminal of the third capacitor is electrically connected to the other of the source and the drain of the fifth transistor.
 7. An electronic device comprising: the imaging device according to claim 1; and a display device, an operation key, or a shutter button.
 8. An imaging device comprising: a photoelectric conversion element; a first transistor; a second transistor; a third transistor; a fourth transistor; a fifth transistor; a sixth transistor; a seventh transistor; a first capacitor; and a second capacitor, wherein one terminal of the photoelectric conversion element is electrically connected to one of a source and a drain of the first transistor, wherein the other of the source and the drain of the first transistor is electrically connected to one of a source and a drain of the seventh transistor, wherein the other of the source and the drain of the first transistor is electrically connected to one terminal of the first capacitor, wherein one of a source and a drain of the second transistor is electrically connected to the other terminal of the first capacitor, wherein the other terminal of the first capacitor is electrically connected to one terminal of the second capacitor, wherein one of a source and a drain of the third transistor is electrically connected to the other terminal of the second capacitor, wherein the other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor, wherein one of a source and a drain of the fifth transistor is electrically connected to the one of the source and the drain of the fourth transistor, wherein a gate of the fifth transistor is electrically connected to the one of the source and the drain of the third transistor, and wherein one of a source and a drain of the sixth transistor is electrically connected to the other of the source and the drain of the fifth transistor.
 9. The imaging device according to claim 8, wherein the other of the source and the drain of the second transistor is electrically connected to the other terminal of the photoelectric conversion element.
 10. The imaging device according to claim 8, further including a fourth capacitor, wherein one terminal of the fourth capacitor is electrically connected to one of the source and the drain of the second transistor.
 11. The imaging device according to claim 8, wherein each of the first to seventh transistors includes an oxide semiconductor in an active layer, and wherein the oxide semiconductor includes In, Zn, and M, where M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf.
 12. The imaging device according to claim 8, further comprising a third capacitor, wherein one terminal of the third capacitor is electrically connected to the other terminal of the second capacitor, and wherein the other terminal of the third capacitor is electrically connected to the other of the source and the drain of the fifth transistor.
 13. An electronic device comprising: the imaging device according to claim 8; and a display device, an operation key, or a shutter button.
 14. An imaging device comprising: a photoelectric conversion element; a first transistor; a second transistor; a third transistor; a fourth transistor; a fifth transistor; a sixth transistor; a first capacitor; and a second capacitor, wherein one terminal of the photoelectric conversion element is electrically connected to one of a source and a drain of the first transistor, wherein the other of the source and the drain of the first transistor is electrically connected to one terminal of the first capacitor, wherein one of a source and a drain of the second transistor is electrically connected to the other terminal of the first capacitor, wherein the other terminal of the first capacitor is electrically connected to one terminal of the second capacitor, wherein one of a source and a drain of the third transistor is electrically connected to the other terminal of the second capacitor, wherein the other of the source and the drain of the third transistor is electrically connected to one of a source and a drain of the fourth transistor, wherein one of a source and a drain of the fifth transistor is electrically connected to the one of the source and the drain of the fourth transistor, wherein a gate of the fifth transistor is electrically connected to the one of the source and the drain of the third transistor, and wherein one of a source and a drain of the sixth transistor is electrically connected to the other of the source and the drain of the fifth transistor.
 15. The imaging device according to claim 14, wherein the other of the source and the drain of the second transistor is electrically connected to the other terminal of the photoelectric conversion element.
 16. The imaging device according to claim 14, further including a fourth capacitor, wherein one terminal of the fourth capacitor is electrically connected to one of the source and the drain of the second transistor.
 17. The imaging device according to claim 14, wherein each of the first to sixth transistors includes an oxide semiconductor in an active layer, and wherein the oxide semiconductor includes In, Zn, and M, where M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf.
 18. The imaging device according to claim 14, further comprising a third capacitor, wherein one terminal of the third capacitor is electrically connected to the other terminal of the second capacitor, and wherein the other terminal of the third capacitor is electrically connected to the other of the source and the drain of the fifth transistor.
 19. The imaging device according to claim 14, wherein the photoelectric conversion element is a photodiode, and wherein a potential of the other terminal of the photoelectric conversion element is variable.
 20. An electronic device comprising: the imaging device according to claim 14; and a display device, an operation key, or a shutter button. 